Labour MPs are reportedly circulating a draft email urging Gordon Brown to stand down.
Amid an apparently swelling mutiny against the Prime Minister, there were reports that up to 100 MPs could be prepared to put their names to it.
The email is said to be addressed to Mr Brown, who is already reeling from the announcements of four departures from his Government in the past two days.
The email states: "We are writing now because we believe that in the current political circumstances you can best serve the interests of the Labour Party by stepping down as Prime Minister."
No-one has so far broken cover to claim ownership of, or support for, the email.
Under party rules, 70 of Labour's 350 MPs are needed to mount a leadership challenge to an incumbent.
The email states that its ringleaders will not publish a list of names until it reaches 50 in number, but they hope to secure "significantly more".
It also pays tribute to Mr Brown's "significant contribution" to the country and the Labour Party.
One MP suggested there could be an impromptu vote of no confidence in the Prime Minister when the Parliamentary Labour Party meets on Monday.
And senior backbencher Barry Sheerman warned Mr Brown was "in trouble" if he did not react swiftly to widespread disillusionment in the PLP.
