The clearest reason for forcing all politicians to be open about the sources and extent of their funding is to establish whether there is any relation between the money they receive and the policies they adopt.

Yet in the case of the Mayor, Ken Livingstone, it is difficult to establish the precise source of his funds because donations are made, not to him directly, but to the London Labour Party. Mr Livingstone himself is not registered as a receiver of donations.

This makes it almost impossible to establish who is supporting Mr Livingstone's campaign for reelection. What is more, the rules of disclosure mean that they will only be declared in three months' time - long after the election. This is perfectly legal. It is also at odds with the whole spirit of legislation requiring transparency in funding.

The Mayor of London's influence over planning decisions is already considerable - and will be greater after this election as a result of new legislation. In 2004, two major developers, Gerald Ronson and Irvine Sellar, made personal donations to Mr Livingstone's campaign of just under £5,000, the level at which such contributions have to be made public. And, as we report today, Mr Livingstone devoted a great deal of effort to promoting Mr Sellar's flagship project, the controversial Shard of Glass skyscraper, and used council taxpayers' money to campaign for Mr Ronson's Heron Tower, which was bitterly opposed by English Heritage.

The relationship between Mr Livingstone and property developers, including Messrs Ronson and Sellar, and Tony Pidgley of Berkeley Homes, has always been a mystery. It may be that the Mayor has been perfectly disinterested in promoting their projects and fighting heritage groups and councils to get them approved. But it is important that we know who is funding Mr Livingstone's campaign.

The Tory mayoral candidate, Boris Johnson, has disclosed all donations to his campaign of over £1,000. Mr Livingstone should do no less. His secrecy over donations is contrary to the spirit of the disclosure requirements. The Electoral Commission - which has responsibility over rules governing donations - must without delay put pressure on Mr Livingstone to come clean.

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