Ken-proof, yes, but a powerful symbol on the world stage

The mayor's ability to make changes in local government is limited. But his political clout will be

The problem with the creation of the mayor of London is not that the holder of the post will have too much power, but that he (sorry Susan, but you have no chance) will have too little. That was the oh-so-clever ploy by the Labour hierarchy in order to ensure that control of the capital remained with central government.

The expression "mayor-proof" has quickly become currency among these brilliant arch-plotters who have managed to make such a mess of what should have been a popular and vote-winning initiative. Mayor-proof, said aloud in a "sarf" London accent, sounds like some way of keeping stud horses at bay but it actually describes the way in which the powers of the new mayor and the assembly have been carefully circumscribed. It really means "Ken Livingstone-proof" - the former GLC leader was never far from the minds of those framing the legislation.

Take the biggest issue: transport. And the biggest issue within that: the tube. If genuine devolution had been the basis of the policy, then the government would have waited before pushing through its controversial public-private partnership scheme to fund the much-needed backlog of investment. But no. Instead, it bulldozed the scheme through as part of the Greater London Act (oddly without opposition from Livingstone) to ensure that the plan was "mayor-proof". The new mayor will have no power to change the deal, although there could be a stand-off if Livingstone wins and decides to hold to his existing position of pushing for the alternative bonds scheme.

The PPP was supposed to have been signed and delivered by the time the mayor takes office in July, but now there will be a hiatus which will prove very confusing for the public. London Transport is being broken up, with London Buses being handed straight to the new executive authority, Transport for London, which is under the direct control of the mayor. But because the PPP will not have been finalised, control of London Underground will pass to the Department of Environment, Transport and the Regions until the deal is signed. For the first time since the 1930s, London's public transport will be disintegrated for several months, if not longer. Eventually, although the mayor will have a significant role in transport, he will have to work through TfL, which poses added complexities.

Once you leave transport out of the picture, the mayor has little direct say in the rest of the spectrum of local government. There will be no return to the days of the GLC which, before the Tories dismembered and finally abolished it, owned and ran large swathes of housing and controlled education in central London (through the Inner London Education Authority), as well as running London Transport. The new mayor and the Greater London Assembly are seen as strategic bodies. They will have a staff of 400, compared with the 20,000 who once worked for the GLC. The bulk of the mayor's budget will come from central government grants, which come attached to the various functions being handed over. There will also be a limited power to raise small amounts through additions to the council tax, which will be constrained in two ways: by government threatening to reduce its central allocation if more is raised locally, and by the assembly, which can reject the mayor's budget on a two-thirds majority vote.

In a few respects, there is a genuinely mould-breaking feel to the mayor's role. On policing, for example, the act removes direct control of the Metropolitan Police from the Home Secretary and passes it to a new independent Metropolitan Police Authority. But even here the government has copped out of giving overall power to the mayor: the appointment of the commissioner will still be in the hands of the Home Secretary, unlike police authorities outside London.

Even where the mayor has a strong role, such as in economic development, he will have to work through other statutory bodies. Thus it is the new London Development Agency that has the role of promoting development and regeneration, with funds from central government which previously went to the boroughs.

The mayor will have his fingers in other pies, such as culture, promoting equality and a ragbag of other activities; and here, again, he will be working with committees. He has no education powers.

So the mayor will not be the Tsar of London. Indeed, he will have to look over his shoulder constantly at the assembly of 25 members who are elected for their four-year term at the same time. The assembly is a bizarre construct whose main activity will be to ensure that the mayor does not misbehave, particularly over the budget; but clearly the government thinks there will be enough work to keep assembly members occupied, as they will each be paid £34,438 a year.

The budget looks big on paper - £3.6bn - but in practice the mayor's control of it is minimal. Most of the money comes from central government, and the mayor has limited ability to make radical changes in how it is spent. He could not, for example, decide to chop 5,000 police off the roll, or even stop the police pursuing cannabis smokers and transfer the money to, say, building a new tube line.

The only big potential source of revenues is in transport, through congestion-charging and a levy on workplace parking, and that money will be hypothecated for transport purposes only. Both schemes are more problematic than might at first appear. A road-user charging scheme will be deeply unpopular, even though most Londoners know it is eminently sensible. Just look at how controlled parking zones inflame local passions. The technical aspects are also very complex - no city in the world of the size of London has ever tried to introduce such a scheme - and there will be fierce wrangling about who should be exempted.

Congestion-charging will do little to reduce traffic, as people will simply stump up the money. But it will provide a big income flow. The most important task for the mayor will be to ensure that the money is used on schemes that visibly improve London's transport. Much the same goes for a workplace-parking levy.

It is not just Tony Blair's fear of a powerful Livingstone that has led to constraints on the mayor's powers; the London boroughs have also jealously guarded their existing powers. Take the example of the London Cycling Network. If ever there was a role for the mayor, this is it. Cycling lanes do not end at borough boundaries, and it needs the muscle of a mayor to push through the best possible route, whether the residents of Acacia Avenue, backed by their Nimbyish councillors, like it or not. But the government resisted this approach. While the mayor controls the cycle network budget, he does not have the power to force through particular routes against the wishes of the boroughs; in other words, he cannot fulfil his strategic role in this area. So the cycle network, which in Paris was designed and built within a year by an all-powerful mayor, will remain woefully incomplete and always at risk of being delayed by anti-cycling boroughs such as Westminster.

This example is typical of the compromises made in drawing up the legislation. Indeed the boroughs, in the form of Lord Harris of Haringey, chair of the Association of London Government, recently issued suggestions on the mayor's role in a document that was very clearly a "keep your tanks off our lawn" warning. "Borough councillors", it said, "will still be the elected representatives closest to the people and they will remain as a significant voice through which Londoners will speak." The mayor seems destined to be caught up in rows with the boroughs.

So is the creation of the mayor and the assembly a waste of time? Emphatically not. The mayor will have enormous symbolic power as he will be perceived both at home and abroad as the ruler of one of the world's great cities. The UK has no experience of personality politics, in the way that the US, through its emphasis on a president and state governors, as well as powerful local mayors, already does. The new mayor will be able to say that he was elected by a constituency of more than five million people, over 50 times more than any MP.

The constraints on the mayor's powers will not detract from that symbolic significance. As the stature of the role grows, so will the powers, formal and informal. The post of London mayor - and of the other big town mayors - will change British politics in unpredictable ways. If all the back-stabbing and infighting can be forgotten after the election, it will have brought about a renewed interest in local politics and a genuine decentralisation of power.

Christian Wolmar's book Stagecoach is available in paperback (Orion, £9.99)