The BBC's Tim Franks: "Dobson's campaign team is being investigated by the Data Protection Registrar"

Registrar Elizabeth France told the BBC that she had received a handful of complaints about the use of party membership lists in order to enable Mr Dobson to canvass support for his campaign.

Labour has insisted that there was no breach of the data protection rules and that MPs and MEPs had access to the lists.

But the use of the lists has been seen as giving Mr Dobson an unfair advantage over his rivals for the Labour nomination, Ken Livingstone and Glenda Jackson.

Ken Livingstone: The left-winger is running against Mr Dobson

Mrs France said: "We've now had a number of individuals who have written to us saying that they consider that it is unfair that their names have been used and they are actual recipients of the mailing.

"That now means that I have a duty to investigate that so we have to begin by looking in more detail where that information come from, how it was handled, and whether there has in fact been a breach of the 1984 Act."

She said it was unlikely that a criminal offence had taken place.

Instead, her investigation would consider a breach of one of the data protection principles, whether the use of the information was "unfair".

Mrs France said it was also unlikely the investigation would delay Labour's selection of a candidate.

Mr Dobson's campaign manager Nick Raynsford, Labour MP for Greenwich and Woolwich, said the campaign team had "firm legal advice that our use of Labour's membership lists is perfectly lawful".

He said: "We are pleased that the registrar accepts there is no evidence of any breach of the law.

"We will obviously co-operate fully with any investigation."

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House of Commons leader Margaret Beckett said she found the matter "faintly bizarre".

She said: "I would be absolutely astonished if people who joined the Labour Party didn't think they would get correspondence from people in the Labour Party."

Mr Dobson has called on Labour to allow his rival Mr Livingstone to stand against him in the battle to become candidate for mayor.

He told The Times he wanted to defeat Mr Livingstone in an open contest, and that "it would be better if he ran, from everybody's point of view".

Mr Dobson's remarks follow reports that Labour is determined to block Mr Livingstone when the party's national executive prepares the shortlist for its candidate in a fortnight.

He also questioned the leadership's decision to opt for an electoral college embracing the unions and MPs, rather than a one-member, one-vote system, to choose the candidate.

Raising his objections to Mr Livingstone as candidate, Mr Dobson told the newspaper that former Greater London Council leader wants to win the contest to ultimately become "leader of the opposition" against Prime Minister Tony Blair.

Mr Blair has already expressed his support for Mr Dobson, as has International Development Secretary Clare Short.

Labour's chosen candidate will battle the Conservative candidate Lord Archer and Liberal Democrat Susan Kramer in the vote next May.