Labour has launched an investigation and "paused" the search for a parliamentary candidate in Croydon amid allegations of voter fraud.
The party's London region cancelled a hustings between the four contenders for the new seat of Croydon East and the meeting in which the winner was set to be chosen.
Ballots cast online for the candidates were also scrapped when some members complained they had been unwittingly signed up to take part using fake email addresses.
Some claimed the membership list, sent to would-be candidates so they could canvass local voters, was interfered with.
London Labour said in a message to members that a hustings, due to take place on Saturday, had been cancelled, and any votes for the four candidates already cast had "been voided".
A Labour party spokesman told the Standard that the process was "paused following receipt of complaints" and an investigation launched.
The new constituency takes in large parts of Croydon Central, which is currently represented by Labour MP Sarah Jones.
The area was previously represented by Tory minister Gavin Barwell, but it is predicted to remain a Labour seat at the next general election.
Meanwhile, the party lost a local by-election in east London on Thursday night as grassroots anger at Sir Keir Starmer's stance on a ceasefire in Gaza continues to grow.
Newham Independents candidate Sophia Naqvi won with 46% of the vote to Labour's 27%.
The result gave the Independents their third councillor after Zuber Gulamussen quit Labour earlier this week.
He joined more than 60 other UK Labour councillors who have resigned over the Labour leader's comments on the Israel - Hamas war.
