The Bangladeshi mayor of Tower Hamlets, Lutfur Rahman, is accused of more than doubling the recommended level of funding for Bengali-run charities.
Opposition councillors say they believe the grants were handed out in return for electoral support.
Lutfur Rahman became Britain's first directly elected Asian mayor in 2010, winning as an independent with only 13% of registered voters.
Almost two thirds who turned out to vote were from the Bangladeshi community.
Now Labour and Conservative opposition councillors are alleging that his selection of Bangladeshi charities for grant funding in the run up to the election was motivated by electoral advantage.
The mayor denies the allegations.
The Panorama investigation, The Mayor and Our Money, will air tonight at 8.30 on BBC1.