Ken Livingstone has called in police after claiming an insider illegally hacked into a City Hall system to steal e-mails sent by his race adviser, the London Paper has reported.
Lee Jasper, 49, has been at the centre of controversy over e-mails he sent to charity worker Karen Chouhan, 50.
During a radio interview, Livingstone claimed that the "intimate" e-mails weren't leaked but that systems were hacked.
Scotland Yard confirmed it had received an allegation of misuse of computer material on December 18 last year. Southwark Police is investigating the claim, with assistance from the Specialist Computer Crime Unit.
The publication of the e-mails led to Jasper's resignation and came after months of controversy over his role in funding community groups.