As you hunker down for the last few days of the Camp for Climate Action, discussing how to force your way into Kingsnorth power station in an attempt to prevent the construction of a new coal facility, cast your eyes around your fellow protesters. Do they look entirely bona fide to you? And don't look for the old-school special branch officers - Kent Police are a tiny force. It's the corporate spies hired by private companies you need to watch out for.

According to the private espionage industry itself, roughly one in four of your comrades is on a multinational's payroll.

Russell Corn, managing director of Diligence, one of a growing number of "corporate intelligence agencies", with offices high in the Canary Wharf glass tower, says private spies make up 25 per cent of every activist camp. "If you stuck an intercept up near one of those camps, you wouldn't believe the amount of outgoing calls after every meeting saying, 'Tomorrow we're going to cut the fence'," he smiles. "Easily one in four of the people there are taking the corporate shilling."

In April this year, for instance, the anti-aviation campaign network Plane Stupid, one of the main organisers of the eco-camp built to protest against the expansion of Heathrow Airport, announced that one of its activists, Ken Tobias, was actually called Toby Kendall, was working for a corporate espionage firm called C2i, and had been leaking information about the group to paying clients and the media. He had been hired by an as yet unknown private company to provide information and disrupt the group's campaigning.

When Tobias first turned up at Plane Stupid's meetings in July 2007, he seemed a committed former Oxford student dedicated to reducing aircraft emissions. The group gradually became suspicious because he showed up early at meetings, constantly pushed for increasingly drama tic direct action and - the ultimate giveaway - dressed a little too well for an ecowarrior. When they showed his picture around Oxford they found an old college pal who identified him as Toby Kendall. A quick Google search revealed his Bebo page with a link to a corporate networking site, where his job as an "analyst" at C2i International, working in "security and investigations", was pasted in full public view.

Just a month earlier, a woman called Cara Schaffer had contacted the Student/Farmworker Alliance, an idealistic bunch of American college students who lobby fast-food companies to help migrant workers in Florida who harvest tomatoes. Like the cockle-pickers of Morecambe Bay, many of these workers are smuggled into the US by gangs which then take their passports and force them to work without pay to clear often fictitious debts to regain their papers.

Digging up dirt

Again, Schaffer's excessive eagerness aroused suspicion, and again, the internet revealed her true identity. She owned Diplomatic Tactical Services, a private espionage firm which had pre viously hired as a subcontractor one Guillermo Zara bozo, today facing murder charges in Miami for his role in allegedly executing four crew members of a chartered fishing boat, an allegation he denies. Schaffer turned out to be working for Burger King - the home, perhaps appropriately, of the Whopper.

The cute thing about these two bozos is that they got caught pretty early on, but that was because they were young and had no background in espionage.

The real market is in proper, old-school spies who are suddenly entering the private sector. For professional spooks, the 1990s were no fun at all. The Cold War was over, defence spending was down and a detailed knowledge of cold-drop techniques in central Berlin was useless to governments looking for Arabic speakers who knew the Quran.

From New York and London to Moscow and Beijing, any decent-sized corporation can now hire former agents from the CIA, FBI, MI5, MI6 and the KGB. The ex-spooks are selling their old skills and contacts to multinationals, hedge funds and oligarchs, digging up dirt on competitors, uncovering the secrets of boardroom rivals and exposing investment targets. They are also keeping tabs on journalists, protesters and even potential employees.

"MI5 and MI6 in particular have always guided ex-employees into security companies," explains Annie Machon, the former MI5 agent who helped David Shayler blow the whistle on the security services back in 1997. "It's always useful to them to have friends they can tap for info or recruit for a job that requires plausible deniability. The big change in recent years has been the huge growth in these companies. Where before it was a handful of private detective agencies, now there are hundreds of multinational security organisations, which operate with less regulation than the spooks themselves," she says.

Corn's company Diligence, for instance, was set up in 2000 by Nick Day, a former MI5 spy, and an ex-CIA agent, Mike Baker. Before long, the duo had built up a roster of high-paying clients including Enron, oil and pharmaceutical companies, as well as law firms and hedge funds. In 2001, a small investment by the Washington lobbying company Barbour Griffith & Rogers propelled their growth. However, BGR and Baker sold their stakes in 2005, shortly before a scandal shook Diligence. KPMG, the global professional services firm, accused Diligence staff of impersonating British spies to gain information on a corporate takeover for a Russian telecoms client called Alfa Group. Diligence settled the lawsuit without admitting liability.

Since then, it has recruited the former Conservative Party leader Michael Howard as chairman of its European operations. And it is that sort of respectability and lobbying power that big players are after. In 2007, the parent company of the US private military firm Blackwater, which hit the headlines for gunning down Iraqi civilians in Baghdad last September, entered this market through Total Intelligence Solutions (TIS), a new CIA-type private operation, to provide intelligence services to commercial clients.

Discreet investigations

Blackwater's vice-chairman, J Cofer Black, who runs TIS, spent three decades in the CIA and the state department, becoming director of the Counterterrorist Centre and co-ordinator for counter terrorism, a job with ambassadorial rank. He describes the new company as bringing "the intelligence-gathering methodology and analytical skills traditionally honed by CIA operatives directly to the boardroom. With a service like this, CEOs and their security personnel will be able to respond to threats quickly and confidently - whether it's determining which city is safest to open a new plant in or working to keep employees out of harm's way after a terrorist attack."

Black also says TIS will operate a "24/7 intelligence fusion and warning centre" that will monitor civil unrest, terrorism, economic stability, environmental and health concerns, and information technology security around the world.

The established firms already operating in this area include Kroll, Aegis, Garda, Control Risks, GPW and Hakluyt & Co. More firms are opening every day and there is little regulation of the sector.

Hakluyt & Co was founded in 1995 by former British MI6 officers, with a reputation for discreet and effective investigations. The company butler, a former gurkha, greets visitors to its London HQ, a town house off Park Lane. In winter, meetings can be conducted beside the fire. Computers are rarely in sight. Hakluyt's advisory board has become an exit chamber for captains of industry and former government officials. Members have included Sir Rod Eddington, a former BA CEO, and Sir Christopher Gent, former chief executive of Vodafone.

"It is hard to work well for an oil company without knowing who all the key decision-makers in a government are and having the right contacts to reach them," explains Stéphane Gérardin, who runs the French private security company Géos. "We have an intelligence section where we employ some investigative journalists, people from the finance sector, from equity banks and some from security backgrounds.

"It is an important part of image protection for our clients as well. We have our own tracking and monitoring centre, with analysts doing risk mapping and preparing our clients for every potential problem. It could be about alerting them to local sensitivities. Or, in this globalised internet age, it can be a group of students in Cambridge who have launched a protest website, who may be sending out a petition.

"So we need to be able to understand and prepare our own propaganda to counter such attacks. This is work we do to protect our clients."

Trusted friend

Like the state security services, which ended up running Class War in the 1990s after a hugely successful penetration, these spies work to become reliable members of any protest movement. In April 2007, the Campaign Against Arms Trade called in the police after court documents showed that the weapons manufacturer BAE Systems had paid a private agency to spy on the peace group.

BAE admitted that it had paid £2,500 a month to LigneDeux Associates, whose agent Paul Mercer - accepted as a trusted member of the campaign - passed information, including a legally privileged document, to BAE's director of security, Mike McGinty.

Unlike the security services, however, these services don't bother with penetrating the far left or anti-fascist groups. Their clients are only interested in the protest movements that threaten corporations. And as that is the nature of much protest in these times, it is a wide field, but with a particular impact on environmental groups.

At any of this summer's green protests the corporate spies will be there, out-of-work MI5 agents tapping green activists' mobile phones to sell the information on to interested companies.

Russell Corn knows of incidents where a spook at a meeting has suggested a high-street bank as a target, then left the meeting to phone the officers of said bank, telling them that he has penetrated an activist camp planning an attack and offering to sell the details. Corn has no time for such behaviour, however.

"The thing about a really good private spy," he tells me, "is that you'll never know he's around and he'll never get caught.

"The fact you can't see them . . . it means nothing at all."

"War plc: the Rise of the New Corporate Mercenary" by Stephen Armstrong is published by Faber & Faber (£14.99)

Spooks for hire

Alyssa McDonald offers tips on how to protect your business

Unsure a potential employee is the person for the job?

Try Géos's "violence assessment and prevention" service. "Security Experts and Board-Certified Forensic Psychologists" will help keep potential troublemakers out of your company. Previous clients include the CIA, the FBI and various Fortune 500 companies.

Worried your business may be undercut?

"Commercial and competitive intelligence" services from Diligence can help. These will identify potential rivals, their respective strengths and weaknesses, their allies in commerce and government; they will assess their strategies, vet potential suppliers and "identify and counteract any rival effort to weaken [the client's] reputation".

Business at risk from radical activists?

Then you need "close protection analysis" from Diligence. A "prominent European scientific research group" suspected of animal testing used the service to provide early warning of possible attacks by animal rights activists. Diligence identified factions within the activist organisation with differing opinions about the use of violence in their campaigns. Playing on this friction, Diligence learned about upcoming attacks and warned the research group.

Concerned that your flight might not go smoothly?

Géos has an online database tracking air carriers' financial position, maintenance practices and history of accidents, as well as their pilots' "training, background and experience". Also available for helicopter services.

Need to find out who your colleagues have been emailing?

Kroll's "computer forensics" can help. They can uncover lost or hidden files, break encrypted files and "re-create events from electronic footprints". Kroll will work onsite if necessary, "even in the middle of the night, so that users are unaware" of what is happening to their computers.

Need to make a sharp exit?

Why not sign up for Géos's "emergency evacuation support" system? Dangers that you may face will be mapped by the company's "intelligence division and global monitoring programme". When security conditions "escalate to a potential crisis point", you can be removed from whatever situation you are in.