Elizabeth France, the Data Protection Registrar, is to hold an inquiry after receiving six written complaints about Mr Dobson using the lists to contact party members at home.
The lists have been seen as giving Mr Dobson an unfair advantage over his main rival, Ken Livingstone, in the increasingly bitter race for the nomination.
Mrs France said it was "unlikely" there had been a criminal offence, although the principles of the Data Protection Act could have been broken. Mr Dobson's camp said it had "firm legal advice" that the use of Labour lists was "perfectly lawful".
But Shaun Woodward, the Tory spokesman on London, said: "Labour's heavy- handed attempt to try to block Ken Livingstone by contacting Labour members on behalf of Frank Dobson has now thrown the entire mayoral selection into chaos.
"It is a great pity that Labour's tactics now run the risk of damaging the entire image of the selection of London's mayor."
Glenda Jackson, one of the other two candidates for the Labour nomination, said she would not accept the nomination unless she won the constituency section in the electoral college.
The move follows Mr Dobson's comments that he would have preferred the candidate to be chosen by one member, one vote, rather than the electoral college drawn up by Labour's ruling National Executive Committee.
Later, Mr Dobson confirmed that he wanted Mr Livingstone to join the shortlist of candidates.
"I have always believed it was right and proper that Ken should be on the ballot paper," he told BBC Radio 4's The World at One.Reuse content