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Labour MP Chuka Umunna has accused leader Jeremy Corbyn of driving centre-left MPs like himself out of the party.
The MPa leading member of the cross-party People's Vote for a second EU referendumurged Mr Corbyn to "call off the dogs".
In a speech, the MP will say so-called moderates face a "clear and present danger" of being run out of the party by hardline factions.
A Labour Party source called the speech "incoherent and inaccurate".
Some of Mr Corbyn's critics have faced battles with their local parties.
Two of them lost no-confidence votes by local party members in their constituencies this week.
Joan Ryan, MP for Enfield North, and Luton South MP Gavin Shuker dismissed the votes, which carry no official force in the party.
They said they would carry on as usual, with Ms Ryan blaming "Trots, Stalinists, Communists and assorted hard-left" for the 94-92 vote defeat.
Jeremy Corbyn emphasises his party's aspirations following criticism by former PM Tony Blair
It comes as former Prime Minister Tony Blair said he was "not sure it is possible" for Labour "moderates" to take the party back from Mr Corbyn and the leftand hinted at the creation of a new "centre ground" party.
Mr Umunna has written to members of his local Labour Party in Streatham, south London, to deny reports that he is involved in talks about a new party, the New Statesman reports.
In a speech to the Blairite campaign group Progress on Saturday, the former frontbencher is expected to say MPs are being targeted for standing up for zero tolerance of racism.
He will say: "My message to our leadership: it is within your power to stop this, so call off the dogs and get on with what my constituency, one of the most diverse communities in the nation, demands we dowithout equivocation, fight this Tory Brexit.
"That is where all our efforts should be."
Mr Umunna will tell the BAME Voices for Progress conference that the Brexit debate has normalised hatred and that black and minority ethnic voters have "paid the price".
He will warn the Labour leadership it would be a "complete betrayal" of the party's values to "act as a bystander and wave through this disastrous Brexit" and call for it to back a referendum on the final deal.
A Labour Party source criticised the speech.
They told the BBC: "This is incoherent and inaccurate, and does a disservice to his constituents who want Labour to oppose the Tories' brutal cuts to the services our communities rely on."
Others Labour MPs have lost local confidence votes include Frank Field, who quit the parliamentary party last weekciting the handling of the anti-Semitism row and bullying in local constituency partiesand Kate Hoey.
Graham Stringer won a confidence vote in his constituency.
These ballots were organised by activists angry at Mr Field, Ms Hoey and Mr Stringer's decision to back the government in Brexit votes, which they say robbed Labour of the chance of forcing a general election it could have won.