On the eve of entering 10 Downing Street as Prime Minister, David Cameron was convinced that his bid to oust Gordon Brown had failed and the Conservatives would remain in opposition, it has been revealed.
Mr Cameron was caught by surprise by Mr Brown's decision to step down, and had to ring wife Samantha and tell her to "get your frock on" as they were going to Buckingham Palace.
And he revealed that when the Queen asked him to form a government, he was unable to say what kind of administration he could construct.
The revelations came in a BBC Two documentary on the dramatic five days following the inconclusive result of the May 6 General Election and the frenzied negotiations over the formation of a new Government.
After four days in which Mr Cameron's overtures appeared to be paving the way for coalition with the Liberal Democrats, Westminster was shocked on Monday May 10 by Mr Brown's announcement that the third party had asked to open talks with Labour.
Mr Cameron told the documentary Five Days That Changed Britain that at this point he believed he would not be Prime Minister.
Mr Cameron said: "I remember saying, 'It's not going to happen, I'm going to be leader of the Opposition'.
"And I remember saying 'I want to go on being leader of the Opposition. I think we can defeat this new government. I'm depressed that it hasn't worked out as we wanted it, but we've got a strong party, had a strong election, we did very well, we made the right offer to the Lib Dems... we behaved in the right way, we made them a big generous offer to have a coalition government. They're going with the other lot, we're going to fight them all the way'.
"On Sunday I was thinking I probably will be Prime Minister... I was thinking by the end of Monday I definitely won't be Prime Minister."
Mr Cameron recalled his surprise when Mr Brown announced at 7.20pm on the Tuesday that he was going to the Palace to offer his resignation to the Queen. "I remember having to ring Samantha and saying, 'What are you doing?'," he said. "And she was doing Nancy's homework and I said, 'We could be going to the Palace, you'd better get your frock on'."