Labour MP Glenda Jackson says she is prepared in principle to act as a "stalking horse" to trigger a leadership contest against Tony Blair.
Ms Jackson says the prime minister should quit soon
The former transport minister said she could run if Mr Blair sought to stay in power for more than another year.
But the Hampstead and Highgate MP said it was "extremely unlikely" she would get the required number of votes.
Another backbencher - John Austin - has previously indicated a willingness to stand as a "stalking horse".
Ms Jackson, a former transport minister, opposed the war in Iraq.
She said: "If there was a group of MPs who asked me to put my name forward, I would be ready to be a stalking horse.
"My feeling is, if he is not going to go now, then he should go at the end of the first 18 months of this Parliament, at the time of the Queen's Speech next autumn.
"Ideally, he would go now, because we have got local elections to fight."
Ms Jackson has been MP for Hampstead and Highgate, in London, since 1992.