Mr Livingstone’s chances were damaged when it emerged that he was being paid through a company which could reduce his tax bill.
All three party leaders have suggested that they are happy to publish their tax returns but so far none has committed to taking the first step.
Last April Mr Cameron, the Prime Minister, made clear he was “very relaxed” about publishing his tax returns and believes the “time is coming” for politicians to be more open about their personal finances.
George Osborne, the Chancellor of the Exchequer, also said the Government would “think through” making senior politicians reveal their tax returns but that he would be “very happy” to consider the proposal.
Nick Clegg, the Deputy Prime Minister also said at the time it was a “matter of principle that the public has a right to know” about politicians’ financial affairs.
Ed Miliband, the Labour leader, said he would be happy to publish his tax return, adding he felt the enhanced transparency was “inevitable”.
The Government was reported to be ready to publish suggestions of how to make the returns public over a year ago, however no details were published.
Earlier this year Mr Clegg appeared to back down from making his tax return public: “My tax return is fantastically dull. I have one income and that is published.”
Pressed again, Mr Clegg added: “No, look, I will do what everyone [does], I am totally relaxed about my own tax arrangements.”
A source close to Mr Clegg added that the Liberal Democrats had no objection to it being done “but it probably makes more sense to do it as a Government rather than individually”.
Figures last year suggested Mr Cameron is worth almost £4million.
Wealth-X, a consultancy specialising in analysing the financial affairs of US politicians, estimated Mr Cameron’s net worth at £3.8million, with liquid assets of £190,000 from current and previous salaries – half as much as President Barack Obama.
Mr Clegg was estimated to have a fortune worth £2million, while Labour leader Ed Miliband and his brother David are worth £1.9million each.