Len McCluskey, a key ally of Jeremy Corbyn, said MPs now had to “respond to local party members” whereas before some acted like they had a “job for life”.
The Unite boss told the BBC: “Well now, in Corbyn’s Labour, people have to respond to their local party members.
“Now, if the local members decide they don’t represent them anywhere and use the appropriate procedures, they should leave and I won’t cry over it.”
Tom Watson told Sky News that Mr McCluskey’s remarks were unhelpful and a distraction for the party.
“Every Labour MP is selected by their local party to stand at every general election,” Mr Watson said.
“If he’s saying that he thinks we should start removing hard-working Labour MPs from office then I think that would be a mistake.
“I think it would be a diversion from our ability to campaign against Tory austerity which is what I assume Mr McCluskey wants Labour MPs to be doing.”
This is not the first time Mr McCluskey has taken aim at critics of Mr Corbyn.
Last month he wrote an article in the New Statesman accusing MPs of “working overtime” to portray the party as a “morass of misogyny, anti-Semitism and bullying”.
Mr McCluskey claimed Chris Leslie, Neil Coyle, John Woodcock, Wes Streeting and Ian Austin were “polluting” Mr Corbyn’s bid to tackle the party’s anti-Semitism row.