Ken Livingstone’s previous campaign to be re-elected as Mayor of London was financed by a £20,000 donation from the train drivers’ union Aslef that does not appear in the Electoral Commission register, The Times can reveal.

Commission rules say it must be registered as a donation to Mr Livingstone if it “specifies that it is for the benefit of a particular candidate”.

The Aslef accounts reveal that in 2003 the union made a £20,000 donation to “Mayor of London Electoral Campaign” – yet nothing appears by Mr Livingstone’s name on the commission website. The revelation has prompted further questions about how Mr Livingstone is able to raise money without having to make donors public by asserting the money goes to the central party coffers.

The Conservatives say that Aslef has made commuters’ lives a misery through industrial action.

A Labour Party spokesman said that the mayoral campaign was run by the London Labour Party which was why it did not appear in the Electoral Commission register, and that the Electoral Commission had approved the arrangement as far back as 2004. The spokesman said: “The Labour Party is pleased that the Electoral Commission has today made clear that there has been no breach of party-funding rules by the London Labour Party or Ken Livingstone in respect of donations to the London election campaign.”

There was further confusion last night because the £20,000 Aslef donation does not appear to have been put on the party’s national register either, although the union gave 13 separate donations totalling £53,914.

The Labour Party denies that any donations have not been declared properly.

Greg Hands, Conservative MP for Hammersmith and Fulham, said: “It is outrageous that London voters do not know who is funding Ken Livingstone’s expensive campaign for reelection. The whole point of these rules is to allow transparency – yet Livingstone hasn’t declared a single donation given to him since 2000.

“The £20,000 Aslef donation wasn’t even declared by the Labour Party,

“ which shows that their arrangements for reporting donations made to Ken Livingstone are utterly flawed. In the public interest, he should release details of his donations right now.”

The Electoral Commission confirmed that it had forced Mr Livingstone to take down a form soliciting donations which asked for cheques to be written to the “Ken Livingstone campaign”.

Outlining his plans on London’s environment, Boris Johnson, Mr Livingstone’s Conservative rival, said yesterday that householders should be paid to recycle waste and pledged the planting of 10,000 trees at roadsides.

His proposals were ridiculed by Mr Livingstone, who accused his Tory opponent of “lacking any credible policies” to tackle climate change and being a member of the George Bush club of politicians who had failed to support the Kyoto pact on climate change.

Mr Johnson, saying that he wanted to make London the greenest city in the world, invoked a recycling scheme in US towns and cities whereby people are paid for the weight of material they recycle. By working with London boroughs, he said, recycling schemes could be organised so that residents were rewarded helping to achieve reductions in landfill waste.

In his mini-manifesto on the environment Mr Johnson, MP for Henley, emphasised the need for clean, green open spaces and pledged to spend £6 million on improvements. He promised to work to stop developers concreting over gardens with blocks of flats and mini-estates, and to protect the green belt.

Times Recommends

• Margot Fonteyn: In her footsteps
• Forget beautiful people, let's have wonky Brits
• Fundamentalist brew: faith and power