A ballot box in the candidate election was found to have a broken seal
The Labour Party has called off a hustings to choose a parliamentary candidate after discovering a ballot box had apparently been tampered with.
The contest in the Erith and Thamesmead constituency in south east London had already provoked controversy when it was taken over by the central party.
That led to complaints that non-local candidates were being favoured.
A London Labour spokesman said Saturday's hustings had been postponed and an investigation had begun.
Sitting MP John Austin, who is retiring, has previously lodged a formal complaint with Ray Collins, the party's general secretary, over alleged rule breaches by campaigners encouraging local members to sign up for postal votes.
A London Labour spokesman said: "It was discovered that the seal on a ballot box containing previously received ballot papers for the selection of Labour's parliamentary candidate for Erith and Thamesmead was broken.
"In order to maintain the integrity of the process, [Saturday's] hustings meeting has been immediately postponed and a new date will be fixed."
The candidates include Georgia Gould, the 22-year-old daughter of Lord Gould, a key aide to Tony Blair during his time as prime minister.
The other seven people hoping to be Labour's candidate for Erith and Thamesmead at the next election are: former minister Melanie Johnson, Greenwich councillor Angela Cornforth, Kensington and Chelsea councillor Marianne Alapini, former Bexley councillor Teresa Pearce, Unite union official Rachael Maskell, Greenwich councillor Jagir Sekhon and Emily Bird.
Whoever wins will be contesting one of Labour's safer seats, inheriting a Labour majority of more than 10,000, even allowing for boundary changes.
The BBC's Guy Smith said it had been a "controversial" selection process due to claims of postal voting "irregularities", but also because of "allegations of nepotism" regarding Miss Gould.
He said: "There is local resentment that she has been parachuted in by Labour's high command."
Our correspondent said some were concerned she may be "too youthful" to represent one of the most deprived areas in London, and there were questions over whether she had "any connection with local people".