The Tory MPs voted in a secret ballot by 127 votes to 50 in favour of giving themselves the final say over the election of the new leader. Michael Howard will resign as leader on 7 October, the day after the annual party conference in Blackpool, triggering the hustings that could produce a new leader by 10 November.
A former cabinet minister said: "I think it will bring Ken in, because he would not go in front of the membership, having been beaten last time by Iain Duncan Smith. He now will think that, in a vote among the MPs, he could win."
Mr Clarke, who has beendefeated in leadership bids twice before, repeated his hint this week that he is seriously considering standing. He gave the hint in a speech to the Tory Reform Group. His allies have become frustrated at his failure to make it clearer, and one of his main allies, Ian Taylor, has indicated he would support David Davis.
Mr Clarke's entry into the crowded field could change the betting. He is bitterly opposed by Eurosceptics but still has the political clout and "bloke appeal" with voters to attract support from Tories who put winning the next election above purity of policy. Under the new system, nominations will close on 20 October, followed by a consultation period lasting until 3 November with ordinary party members and hustings.
The rank and file will be able to submit the list of candidates in their preferred order to MPs who will make the final choice. The first ballot among MPs will be held on 8 November, eventually being whittled down to the final two in a run-off.
Mr Howard, Theresa May, the former party chairman, Michael Ancram, the deputy leader, and the new Tory MP Michael Gove were among those opposed to the changes. The new system has yet to secure the approval of the Tory party convention comprising activists, MPs and MEPs, but it is likely to go through once it has the backing of the Conservative Party board.
Mr Ancram was holding out hope it would be rejected by the convention. "I am sorry it has gone through. I think it is a retrograde step. I think there is a lot more argument to be had about this," he said.
Damian Green, a member of the 1922 Committee executive that drafted the rules, said: "It's ridiculous we will have to wait until November to get a new leader but, given where we are, this is as fast as possible."