Jeremy Corbyn is being urged to “call off the dogs” to stop centre-left MPs being driven out of Labour.
Former frontbencher Chuka Yunnan claims the so-called moderates are being systematically targeted by more hardline factions and now face a “clear and present danger” of being run out of the party.
The pro-EU campaigner will use a speech on Saturday to call on the leadership to stop using internal divisions as an excuse not to fight Brexit.
His intervention comes after Tony Blair attacked Mr Corbyn’s leadership of the Labour Party and questioned whether it can be “taken back” by moderates.
The former prime minister sparked anger among supporters of the current left-wing leader on Friday by saying that under his stewardship it had become a “different party” and he hopes it is not “lost”.
A number of Labour MPs who have been critical of Mr Corbyn, particularly over his handling of the anti-Semitism row, have found themselves locked in battles with members of their constituency Labour party.
Labour Friends of Israel chairwoman Joan Ryan, a former minister under Tony Blair, and Luton South MP Gavin Shuker, both lost local no confidence votes on Thursday.
In a speech to the centre-left group Progress, Mr Umunna said MPs are being targeted for standing up for zero tolerance of racism.
“More motions such as this are expected by colleagues,” he said.
“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 do – without equivocation, fight this Tory Brexit. That is where all our efforts should be.”
Mr Umunna will tell the BAME Voices for Progress conference the Brexit debate has normalised hatred and 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.
Mr Corbyn was in Leicester on Friday to outline Labour’s plans for the water industry.
When told that Mr Blair sees him as an “existential threat” to the party he said: “I’ve been in the Labour party all my life. I am a socialist. I am determined to see a fairer and more equal society for everybody.
“That’s what the Labour party exists for.”
Mr Blair had said his associates “feel that the Labour Party’s lost, that the game’s over. I’m kind of hoping they’re not right”.
It prompted Jon Lansman, founder of the Corbynite Momentum movement to say on Twitter that Labour would “never” return to the former PM’s policies and he “was never in the right party”.
However, Lord Blunkett, a former Home Secretary and leading figure of the Blair era, warned that the party faced “irrelevance” unless there was a rethink of the “Corbyn project”.