Get your frock on, darling, I'm going to be PM

In a BBC documentary of the five frantic days following May's general election, the Prime Minister also discloses that, after being asked by the Queen to form a new government, he was unable to tell her what form it would take, with Liberal Democrat negotiations still taking place. "I might have to come back in the morning," he warned her.

Both the Camerons were also caught out by Gordon Brown's abrupt departure from Downing Street. With Mr Cameron expected to assume a speedy handover of power, he had to call his wife Samantha at home to tell her to "get your frock on".

The programme tonight will also lay bare the tensions between the two coalition partners. Former Lib Dem leader Sir Menzies Campbell says there is a "very grave risk" that Mr Cameron and Deputy Prime Minister Nick Clegg could end up looking like one another politically - even comparing the situation to how an owner ends up looking like its dog.

The first-hand accounts are the most detailed yet of the hours and days following May's result when, for the first time since 1974, voters returned a hung parliament.

The Lib Dems initially negotiated with the Conservatives in the weekend after the result, with Mr Cameron having won far more seats than Labour. But, after talks got bogged down over the question of a referendum on voting reform, Mr Clegg then turned back to Labour.

With Mr Brown having resigned that day, clearing the way for a new party leader to work with Mr Clegg, Mr Cameron recounts how he thought the game was up.

"I remember going home, I think on Monday evening, and I think Sam and I had supper in the kitchen and I remember saying, 'You know, it's not going to happen. I'm going to be leader of the opposition'."

Mr Cameron recounts how, having seen the Lib Dem-Labour talks begin, he concluded that they were "going with the other lot". He goes on: "I thought, 'That's it, I'm going to be in opposition for another couple more years'. On Sunday I was thinking, 'I probably will be prime minister'. On Monday I was thinking … 'by the end of Monday I definitely won't be prime minister'."

Less than 24 hours later, however, Mr Cameron walked into No 10 after the talks between the Lib Dems and Labour floundered that afternoon.

The speed of the transition of government took him by surprise, he reveals. "I remember having to ring Samantha and saying, you know, 'What are you doing?', and she was doing (the Camerons' daughter] Nancy's homework and I said, 'We could be going to the Palace, you'd better get your frock on'."

When the pair arrived, however, the Lib Dems were still to ratify the coalition pact. He says: "I went to see, Her Majesty, and she asked me to form a government and I said I couldn't be totally sure about what sort of government I was going to form. I said that I hoped to form a coalition government, but I might have to come back in the morning and tell her it was something rather different."

The programme suggests there was tension between Buckingham Palace and Mr Brown over the timing of his departure. Lord Mandelson, who is also interviewed, reveals he told the Palace that Mr Brown wanted to leave "swiftly". He had faced headlines over the weekend accusing him of "squatting" in Downing Street.

But, says Lord Mandelson, the Queen's principal private secretary insisted he cling on. "He said, 'Look, I fully understand, nobody likes it but, equally, the Prime Minister has a constitutional obligation, a duty to remain in his post until the Queen is able, and to ask for somebody, either Gordon or an alternative, to form an admin'."

The programme also answers some of the questions about the crucial negotiations over voting reform.

Mr Clegg, who is also interviewed, says he wasn't "formally" offered the promise of reform without a referendum by Labour, as has been claimed. However, Mr Cameron says he was "certain" Labour was offering the Lib Dems such a deal. It was this prospect that allowed him to persuade his own MPs to accept a referendum as the price of a deal.

Nonetheless, Mr Cameron denies misleading his MPs in pursuit of a pact.

Mr Cameron says his good relations with Mr Clegg dated to a chance 45-minute encounter when they were left alone together at a parliamentary function. "I think that helped. I knew that he was a reasonable person, in politics for the right reasons like me," says Mr Cameron.

However, Sir Menzies adds: "You know that old thing if you have a dog then, eventually, for long enough eventually you begin to look like your pet. Well, if you have a coalition partner, then it seems to me there's a very grave risk eventually you'll come to look like them."

•Five Days That Changed Britain will be broadcast on BBC Two at 9pm tonight.