Ken Livingstone’s only declared donation to his last election campaign was a single £356,413 payment from the London Labour Party, prompting fresh questions about transparency.

With six days to go until election day, Mr Livingstone was accused in Parliament of engaging in “dirty politics” by failing to list all the backers of the mayoral election campaign in his returns, seen by The Times.

Donations coming via the Labour Party have to be declared to the Electoral Commission only if they are above £5,000, and the fact that their destination is the Livingstone campaign is never made public.

Greg Hands, the Tory MP for Hammersmith and Fulham, told the Commons he had taken up the issue with the Electoral Commission in March and found their response “wholly unsatisfactory”. The Commission cleared Mr Livingstone of breaking any rules.

This came amid the latest signs that the race for City Hall remains wide open, with a new poll for Mr Livingstone showing him edging ahead of his Tory rival, Boris Johnson.

The Ipsos MORI poll for Unison, the public sector union, puts Mr Livingstone on 53 per cent, six points ahead of Mr Johnson, among those certain to vote in next Thursday’s election, once second preferences are taken into account.

The study found that the Labour candidate is first choice for 41 per cent of voters, three points ahead of Mr Johnson on 38 per cent. Brian Paddick, the Liberal Democrat candidate, is third on 12 per cent.

However, the picture painted by recent polls is inconsistent. The last Ipsos MORI poll for Unison on April 9 had Mr Livingstone one point ahead on first preferences, while a poll by the same company for The Observer two days later had Mr Johnson six points behind. The last YouGov poll on April 18 showed the Tory candidate seven points ahead.

Mr Livingstone’s 2004 election returns, which have not been seen before, reveal that he declared a precampaign £10,000 donation from Aslef, the train drivers’ union, almost seven months after he received it.

It also shows that he spent £10,000 on a booklet, £43,196 on design services and £17,132 on his website, but only £6,232 on staff. The Labour Party said that many people would be working on the campaign without charge.

Mr Hands told MPs that he wanted to see the mayor treated as a regulated donee as defined under the Political Parties, Elections and Referendums Act 2000. This would mean that the public would be able to see who was donating to the campaign before elections on May 1, he said.

“Five million electors deserve to know who is paying for all of the glossy leaflets coming through their doors,” he said.

Mr Livingstone’s supporters say he does not need to register donations because supporters give funds to the Labour Party. Critics say this means that the identities of Mr Livingstone’s backers can legally remain secret until after the election.

But Mr Hands said: “Despite a separate existing bank account for Livingstone donations, despite literally millions of leaflets pouring off the presses, despite widespread media reports of lucrative fundraising options and past evidence existing from his 2000 campaign that he had the ability to raise significant sums of money in his own name, the Electoral Commission could see no evidence that a single donation had actually been made to the Livingstone campaign and therefore could see no grounds for investigation. This is a farce, because of the subterfuge and the refusal to report any donations.”

In a separate election development Matt O’Connor, the candidate for The English Democrats’ party, halted his campaign after a fall-out with the party. The Fathers4Justice founder told BBC London that he had been “sidelined” and complained about the lack of publicity, even on St George’s Day. It is too late for him to withdraw from the election, however, and his name will appear on the ballot paper on Thursday.

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