Iain Duncan Smith was tonight dumped by Tory MPs as their leader after he lost a confidence vote among MPs by 75 votes to 90.
Mr Duncan Smith is now bound to resign publicly, in a statement to be made outside Conservative Central Office.
The result of today's ballot means a bruising leadership contest is almost inevitable.
In a typically dignified speech, Mr Duncan Smith pledged his absolute support for his successor although he would not publicly endorse any single candidate.
Mr Duncan Smith that it had been an "immense honour" to lead the party and added: "I am particularly sorry that I will not have the opportunity to fulfil my promises to people in some of the poorest communities in the country. But loyally from the backbenches, I will continue my campaign for social justice."
One of Mr Duncan Smith's opponents in the confidence vote, Crispin Blunt, said: "It is a sad and serious moment but it is an evening when the Conservative Party became serious about the next General Election."
The nominations for the next leader will close on Thursday November 6. The first ballot will be held on November 11.
The first shock came as David Davis ruled himself out as a contender for the leadership and backed Michael Howard, who was immediately established as cleare favourite for the post.