Labour MPs have overwhelmingly backed a vote of no confidence in Jeremy Corbyn.
The Parliamentary Labour Party voted by 172 to 40 for the motion tabled by veteran backbencher Dame Margaret Hodge, sources said.
But the defiant leader said he will not "betray" his supporters by resigning.
He said the vote by the Parliamentary Labour Party had no "constitutional legitimacy" under party rules.
Although there was no official announcement of the voting figures, sources said that it was 172 to 40 in support of the motion, with four spoilt papers.
In a statement, Mr Corbyn said: "I was democratically elected leader of our party for a new kind of politics by 60% of Labour members and supporters, and I will not betray them by resigning. Today's vote by MPs has no constitutional legitimacy.
"We are a democratic party, with a clear constitution. Our people need Labour party members, trade unionists and MPs to unite behind my leadership at a critical time for our country."