Michael Portillo yesterday tried to steal a march on his rivals for the Tory crown by declaring his candidacy and announcing the backing of two-thirds of the shadow cabinet.
He promised huge changes to the Conservative Party after two "catastrophic" election defeats, and a team spirit approach to leadership.
But he was rebuffed within minutes by Shadow Home Secretary Ann Widdecombe, who declared she could never serve under him as leader.
Ms Widdecombe, who has yet to declare her intentions in the leadership contest, said: "I simply cannot serve Michael Portillo - it is as simple as that.
"The problem would be that little band of back-biters that surrounds him. I don't think I could go through another four years of that."
Her outburst threatened to sour Mr Portillo's big day, as she made plain the contest for the Conservative leadership could be a bitter and personal one.
No other potential challengers were flushed out by Mr Portillo's declaration, which saw him confirmed as the bookies' favourite to take William Hague's job.
The Shadow Chancellor's announcement ended days of speculation about his intentions. He told reporters: "I am putting my name forward to be Conservative Party leader.
"Unless the Conservative Party makes huge changes in its style and in the issues on which it focuses, the party could slip still further in public respect."
Mr Portillo stressed he had learned the lessons of the General Election, adding: "We in the Conservative Party need to show that we are people like other people and that their concerns are our concerns.
"In our demeanour and our tone we must show that we are thoughtful, understanding and moderate."
Kenneth Clarke, the euro-friendly Tory former chancellor, refused to comment on Mr Portillo's candidacy and has already said it would be "a week or two" before he makes up his mind what to do.
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