Forget the philandering footballer and the oxymoronic publicity-shy supermodel. There is a more serious threat to the ability of the press to carry out serious investigations, which stems from a slightly odd alliance between a Belgian brewer and the most powerful regulator in the City of London.

It all goes back to an article in the Financial Times late last November, which reported that Interbrew - the said brasseur belge - was plotting a bid for South African Breweries, a rather impressive global brewer with a near-monopoly in a host of African and eastern European countries.

Documents about the putative takeover had been sent by an anonymous leaker to the FT, Independent, Times, Guardian and Reuters. And for reasons that are unclear to me, the FT was the first to write about them. Anyway, the market was impressed: SAB's shares rose 8 per cent and Interbrew's shares fell by more than 7 per cent.

The documents had the ring of authenticity. The various companies involved were all given the standard, silly code names beloved of the public-schoolboy bankers who run these shows. Thus SAB was "Zulu" and Interbrew was "Ice" (investment banking is just so much more fun if the bankers can pretend to be 007).

In the aftermath of the FT story, Interbrew confirmed its interest in buying SAB. However, it believed that part of the documentation sent to the newspapers was fabricated. The offer price and timetable were not genuine, it claimed.

It asked the media groups to hand over the documents, so that it could track down the leaker and prevent a repetition of the alleged damage to its business. They refused; Interbrew took them to court and has won every round in the legal battle, including the appeal (in a unanimous verdict of the appeal judges). Within the UK system, only recourse to the House of Lords now remains, although I suspect the newspapers may take this one all the way to the European Court of Human Rights.

The judiciary has backed the Belgians because it believes the motive for doctoring and distributing the documents was to create a so-called "false market" in the shares of Interbrew and SAB. At the High Court, Mr Justice Lightman said that the leaker "sought to deceive and manipulate the press to achieve his criminal objective, namely to rig the market". This was a "serious criminal offence", he said. In other words, he and the other judges have taken the view that the public interest in pursuing a criminal investigation outweighs the freedom of the press.

What is probably more significant, however, is that Howard Davies - chairman of the City super-regulator, the Financial Services Authority - takes the same view. The authority has extensive new powers to crack down on insider trading and market manipulation, and is keen to exercise them. So Davies has made it clear that even if Interbrew were not suing for the documents, the authority would demand them, and would also have gone to court if necessary.

I asked him why. "This is timely evidence," said Davies, "and it would be difficult to pursue an investigation without this evidence. We can do some things on the basis of trading patterns, but without the actual evidence it would be difficult. And it does look prima facie as if there was an attempt being made to manipulate the markets."

On a narrow interpretation of the issues, the position of the judges and the FSA is reasonable. Would I man the barricades to protect the rights of insider-trading sleazeballs to make a fast buck from manipulating share prices? No.

But nobody has actually proved that the leaker intended to manipulate the markets. The share prices of SAB and Interbrew did move, but I can think of plenty of other reasons why somebody might leak the documents. To argue about the detail of this case, however, is in a way to miss the big issue, which is that if Interbrew - and the Financial Services Authority, by implication - win, there will be a new and significant deterrent to whistle-blowers.

What worries me is that almost anything that any serious whistle-blower is likely to tell a journalist would have implications for a company's share price. So a conscientious investigative journalist would have to tell a source that he would protect his/her anonymity at all costs, but mutter that there is a possibility that doing so would be illegal if there were any suspicion of market manipulation.

Davies, for example, refused to give me any commitment that he would not demand journalists' notebooks in some circumstances. So the leaker would have to be confident that the hack would be happy to go to prison rather than divulge his or her source - which might cause the less fervent whistle-blower to wobble.

The disappearance of certain kinds of newspaper story - the puffs for imminent bids, for example - would not be so terrible. But the price the FSA asks us to pay for less of the frippery is that there will be fewer legitimate probes by the media into company malpractice (you can argue that the press is crap at conducting these anyway, but that is a different argument).

Davies is the most media-friendly and liberal of bureaucrats, so my instinct was that he would not wish to be seen as the human shield of unaccountable big companies. And he says we should trust him not to pursue journalists and newspapers carrying out probes that are in the public interest. Having known him for the best part of 20 years, I take him at his word. But I would not stake the freedom of the press on the liberal instincts of his successors.

Robert Peston is editorial director of QUEST; www.csquest.com; e-mail rpeston@collins-stewart.com