Legal action could be taken against Labour unless it pauses its leadership contest due to fears of "entryism", one of its MPs warned as the deadline for registering to vote in the election passed.
Graham Stringer joined three other Labour MPs in calling for the race to be halted while the party attempts to weed out bogus anti-Labour voters who have signed up as registered supporters for £3.
But surprise favourite Jeremy Corbyn said the public response to the leadership contest was proof of optimism that a popular, modern Labour Party could defeat the Tories.
The emergence of the left-wing veteran as the apparent frontrunner in the race has led to calls for it to be re-run over allegations that it has been exploited by hard-left groups and political opponents including Conservative activists.
Labour has insisted it is taking action to stop charlatan voters casting their ballots and has already excluded 1,200 members or supporters of another party.
But Mr Stringer said legal action could be taken against the party unless it pauses the process, joining colleagues Barry Sheerman, John Mann and Simon Danczuk.
He told LBC's Shelagh Fogarty: "I think if the election isn't paused that would be a shame and I think probably that people will take legal action because the rules are not being implemented properly.
"People who basically have lied when they say they are Labour Party supporters are going to vote in the election, there's lots of credible evidence out there.
"So I think the Labour Party will end up in the chaos of legal process so I don't think it will go ahead on that basis and it's whether the NEC (national executive committee) take that decision or whether it is stopped by legal action."
He added: "I think that if the rules are not being implemented properly and there is credible evidence that they can't be implemented properly then making any organisation, whether it's the Labour Party or another organisation, abide by its rules is something that its members should be (doing).
"So in that sense I would support legal action - I don't have the money or the legal expertise to do it myself."
But Mr Corbyn, who has surprisingly emerged as the bookies' favourite ahead of rivals Andy Burnham, Yvette Cooper and Liz Kendall, defended the "robust" system.
The Islington North MP said: "The response to the Labour leadership race from party members and supporters, longstanding and new, is proof of the optimism in the possibility of a popular, modern alternative to the Conservative Government.
"All of our focus must be on channelling this extraordinary movement towards a united party that is able to find its confidence and defeat the Conservatives in 2020.
"I particularly want to pay tribute to the Labour Party's staff who have worked so hard to deliver a robust selection system in totally new circumstances. The professionalism and commitment of the party's staff shows our movement at its best."