Elizabeth France, the Data Protection Registrar, suggested that MPs and MEPs who had supplied lists to Mr Dobson's campaign may have acted illegally and said she was ready to carry out an inquiry.
Ms France's comments came as Geoff Martin, convener of the union, London Unison, and a Ken Livingstone supporter, revealed that he was making an official complaint.
The former health secretary came under attack last week when it emerged his campaign team had used confidential lists of the 68,000 Labour party members' names and addresses for a mailshot.
Mr Livingstone and Glenda Jackson claimed that the lists gave Mr Dobson a head start in the race for the Labour nomination. Labour's general secretary, Margaret McDonagh, was last night under intense pressure to use her power release the lists to all candidates to ensure a level playing field.
In her first public comment on the claims, the Data Protection Registrar told BBC Radio 4's Today programme that Labour officials did have the right to use information for electoral purposes.
However, she made it clear that mayoral candidates and their supporters might have breached the Data Protection Act if they were not licensed to distribute members' details.
"That's certainly something we would have to look at. And, indeed, we would have to look at the candidates themselves and whether they were in a legal position to process the information, to create the letters and send them out."
Both Labour's Millbank HQ and Mr Dobson's campaign denied any suggestion that the law had been broken, claiming that the party's registration under the Act covered MPs and MEPs.Reuse content