The former health secretary is facing fresh claims that he has enjoyed an unfair advantage over his rivals.

The BBC's Tim Franks reports: "This allegation raises serious questions"

A Dobson supporter who visited his campaign headquarters in early October claimed to have seen there a centrally-held list of Labour's London membership on a computer screen.

Mr Dobson's campaign has been hampered by questions about its use of the lists of the party's 68,000-strong membership in the capital, his access to which has been seen as a significant advantage over rivals Glenda Jackson and Ken Livingstone.

Paul Lettam: Claims Mr Dobson's team has official list

Opponents suspect that Labour's Millbank headquarters had a role in supplying the lists. Mr Dobson's campaign team insists the details came from sympathetic MPs and MEPs, who were entitled to receive the information from party HQ.

Elizabeth France, the Data Protection Registrar, is examining whether those MPs and MEPs were registered to distribute such information.

The situation has been complicated further by Dobson supporter Paul Lettan, who reports seeing a centrally-held list of the London membership at the Dobson HQ.

"It was definitely the Labour membership mailing list. It has a particular screen ... and it says at the top Labour Party Membership list," he told BBC Radio 4's Today programme.

The Dobson campaign responded that the information provided by MPs and MEPs had been passed on in one computer readable format, and converted to another for the campaign's use.

BBC Political Correspondent Tim Franks: "The Labour contest is still dominated by the process of selection"

Later, Mr Dobson called on Labour to ensure a level playing field, by making the London membership list available to all the party's candidates.

"The use we have been making of membership lists does not breach Labour party rules and according to our lawyers is lawful," Mr Dobson said in a statement.

"But to put an end to the feeling that the present situation is unfair to Ken and Glenda, I now call upon the Labour party to give the full London membership lists to all three of us, or failing that to send out mailings to Labour members on our behalf."

'Perplexed'

But Mr Dobson's plea looks unlikely to be met. A Labour spokesman said it was already committed to making available its mail-out system to distribute two mailings on behalf of each shortlisted candidate, at the candidates' expense.

Those mailshots would take place before balloting for the official candidate, but the party would not provide candidates with centrally-held information on membership.

Ms Jackson, the Labour MP for Hampstead and Highgate and former minister for transport in London, said she was "perplexed" by the apparent ease with which the Dobson campaign had managed to access membership details.

"What is disturbing is ... that someone has been able to access, it would appear, the central computer," she said.

The Tory spokesman on London, Shaun Woodward, MP for Witney, accused Labour of resorting to "unfair and unprincipled tactics" to try to secure Mr Dobson's nomination.

"Whilst it is increasingly obvious that Downing Street will stop at nothing to secure the nomination of Frank Dobson, the Labour party should question the long-term damage that its campaign methods are having on the image of the office of the mayor of London," said Mr Woodward.