A Dobson supporter claimed that on 9 October he saw on a computer screen in Mr Dobson's campaign headquarters a list of party members, which he claimed was a centrally held list of Labour's London membership. Under Labour Party rules, no candidate is allowed access to central lists and Millbank officials deny supplying them to Mr Dobson's campaign.
Last night, Mr Dobson called on Labour chiefs to hand over the lists to his Labour rivals in the mayoral race, but he insisted he had done "nothing improper whatsoever".
Party officials are expected to agree to the former Health Secretary's suggestion on the strict condition all three candidates swear an oath that they will not let the lists fall into the hands of the party's enemies.
Paul Lettan, who had been working in the Dobson campaign HQ, yesterday told the BBC: "It was definitely the Labour membership mailing list. It has a particular screen and it says at the top `Labour Party Membership List'.
"At the time I didn't think anything of it, it was only a few days later when I heard that Glenda Jackson and Ken Livingstone didn't have the membership list that I realised.
"I don't believe Frank Dobson had anything to do with this. It's the kind of thing that goes on at a much lower level, where you get young, enthusiastic supporters who possibly get carried away. But I certainly think that it's wrong. I certainly think it's immoral."
Yesterday, Mr Dobson's campaign manager, the Greenwich MP Nick Raynsford, said that the lists, used for a mail-shot of 68,000 Labour members, did not come from Labour HQ, but from individual MPs and MEPs. But the latest allegations left Labour facing an embarrassing investigation into claims that they might have been supplied illegally.
Mr Raynsford insisted that nothing unlawful had happened and stressed it was incorrect to suggest that lists had been made available to Mr Dobson but not to rivals Glenda Jackson and Ken Livingstone. The details are available to MPs, MEPs, candidates for the Greater London Assembly, constituency secretaries, branch secretaries and membership secretaries.
But a spokesman for Ms Jackson said yesterday: "Mr Raynsford's explanation doesn't hold water. Both Glenda Jackson and Ken Livingstone are members of the Parliamentary Labour Party but Glenda was specifically told by Millbank officials that the central membership list would not be made available to her or her campaign."
Ms Jackson herself made it clear that no one in her constituency party had handed over her membership list.
"No one approached me for those lists, to my knowledge no one approached any official in my party or anyone else within the party and yet those lists were furnished. What is disturbing is ... that someone has been able to access, it would appear, the central computer," she said.
The former transport minister added that no one was "more open, honest and decent" than Mr Dobson but said the suggestions of "dirty work at the crossroads" reflected badly on the party.
Labour insisted that no centrally held records had been sent to any candidate. "I am confident that the Labour Party centrally has not provided lists or records to any candidate," a spokesman said.