A group of 75 hard-left former Labour members, including leaders of the Militant Tendency, have applied to rejoin Jeremy Corbyn's party to help stop "further compromise and retreats" to "pacify the Blairites".
All the applicants have previously been expelled from the party, and among their number are members of the entryist Militant group's editorial board.
They include Tony Mulhearn, one of the leaders of the Militant takeover of Liverpool City Council in the 1980s, former Labour MP Dave Nellist, and Peter Taaffe, whose application is likely to be turned down as he is the general secretary of the Socialist Party.
In August, Mr Corbyn had a public falling out with deputy leader Tom Watson after he complained of "Trotsky entryists" infiltrating the party using tactics employed by Militant in the 1980s.
The Labour leader dismissed Mr Watson's claims as "baseless conspiracy theories" but the deputy leader stuck to his argument.
Mr Taaffe said Mr Corbyn's leadership provided an opportunity to rebuild Labour as a "clearly anti-austerity socialist party" and said he and those also reapplying want to "play their part" in the struggle to transform it.
He said: "It is clear that the pro-capitalist wing of the Labour Party remain determined to undermine, and when possible, remove Jeremy Corbyn.
"There is a real danger that, on the basis of further compromises and retreats in a vain attempt to pacify the Blairites, the opportunity to change Labour could be wasted.
"We want to play our part in the struggle to transform Labour and urge the NEC (National Executive Committee) to aid this process by admitting us, and others who have been similarly expelled or excluded, into membership, and also by deciding favourably on requests for affiliation from any socialist organisation that so applies."
A Labour spokeswoman said: "It is against Labour's rules to be a member of another political party or organisation which has its own programme, principles and policy, or distinctive and separate propaganda, and which is therefore ineligible for affiliation to the party."