Mr Clarke, who is expected formally to declare his intention to stand for the leadership in the next few days, was a consistent critic of the 2003 war, even though it was backed by the Tories under Iain Duncan Smith and his successor, Michael Howard.
It was also supported by Mr Clarke's two main rivals for the Tory leadership - the front-runner, David Davis, the shadow Home Secretary, and David Cameron, the shadow Education Secretary.
One MP who supports Mr Clarke said yesterday: "The fact is that Ken got it right on Iraq and others got it wrong. If he had been Tory leader, the political landscape would have looked very different. We could have made the running on the issue at the general election. Ken wasn't a lone voice. A lot of Tories had doubts but went along with the leadership."
Conservative Party officials admitted they had failed to benefit from the leaking of the Attorney General's legal advice on the war during this year's election because the party leadership had endorsed the conflict. Earlier, Mr Howard said he would not have voted for the war if had known in 2003 what he knew now, but his attempt to get off the hook backfired.
Allies of Mr Clarke - a former chancellor, home secretary, health secretary and education secretary - insist it is legitimate for him to raise the Iraq issue because the continuing problems on the ground in Iraq bear out his good judgement - his main "selling point" to Tory MPs given his experience in four cabinet posts.
Mr Clarke is discussing with advisers whether to devote one of the keynote speeches of his campaign to Iraq. He also intends to attack Tony Blair's claim that last month's terrorist attacks in London were not linked to the war by insisting that the conflict increased the threat to Britain.
It emerged at the weekend that the Foreign Office warned more than a year ago that the invasion was fuelling Muslim extremism.
Last week, Mr Cameron said of the war: "I thought then [in 2003] that, on balance, it was right to go ahead, and I still do now."
The Muslim Council of Britain criticised Mr Davis for voting for the war after he urged the Government to scrap its "outdated" policy of multiculturalism.
The only other potential candidate to oppose the war is Sir Malcolm Rifkind, the former foreign secretary, whose leadership campaign may be undermined by Mr Clarke's decision to run.
Sir Malcolm told BBC Radio 4 yesterday: "If the Conservative Party wants to choose the 'one nation' tradition of Conservatism as the way in which it will regain the confidence and support of the public,I would be proud to be its leader."
He added: "If the party, in its wisdom, comes to the view that a more right-wing policy concentrating on immigration, on Europe and issues of that kind are the way forward then obviously it should look elsewhere. I think that's a real choice and I think it's important that in appealing to the public, the party should first be clear what its own priorities are."
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