Postal vote-rigging may have influenced the outcome of local elections in London, a Metropolitan Police Authority (MPA) report has revealed.
Tower Hamlets is under investigation
Scotland Yard Assistant Commissioner Andy Hayman said minority communities were most vulnerable to the corruption.
This comes after police launched 30 investigations into alleged offences after the local elections in May.
Mr Hayman said there was evidence of postal vote fraud in Tower Hamlets, east London.
Truly secret ballot
In the report published on Monday, Mr Hayman said people in minority communities are most at risk because the postal voting forms appear to be so complex.
There is also anecdotal evidence of senior family members instructing relatives to vote for a specific party or candidate.
Mr Hayman said: "Postal voting increases this risk as the safeguard of a truly secret ballot is removed."
Scotland Yard's special prosecution unit (SPU) has, or is conducting, 30 investigations into alleged offences across London, including Tower Hamlets.
In the report for the MPA, Mr Hayman said: "It is the view of the SPU that widespread use of postal votes has opened up a whole new area to be exploited by the fraudster, and the opportunity has been taken.
"It is difficult to assess if abuse of postal voting has altered the outcome of local elections, but it is possible."
He went on to say the postal voting system must be subject to proper management and scrutiny "otherwise the integrity of the electoral system will be compromised".
In communities where English is a second language, people often seek assistance and this can lead to interference from a third party, he said.
The report said there was evidence such activity had taken place within the Bengali community in Tower Hamlets.