David Cameron has told how his son wore tartan pyjamas on the night of the Scottish independence referendum and joined him for the results from 4am.

He confided to a Scottish Conservative event about his sleepless night as he awaited to find out if he would wake up as Prime Minister of the United Kingdom, adding his children knew how worried their dad was that the 307-year-old union could end.

But he said Tory activists had contributed to the "happiest two hours" of his year after they helped secure a no vote.

Mr Cameron told a fringe event at the Conservative Party conference in Birmingham: "I feel very emotional being here tonight because it was just such an important thing that all of you did - our 307-year-old union reaffirmed so brilliantly with so much hard work by all of you.

"You gave me definitely the happiest two hours of my year. I tried to go to sleep, it wouldn't work and I sat at three in the morning in the press office of Number 10 Downing Street and at about four o'clock my children came down - Nancy and Elwen.

"Elwen was wearing some tartan pyjamas. They knew how worried their dad was and they cared deeply about it too.

"When we watched result after result come in and I must say I now know the definition of relief - going to bed as Prime Minister of the United Kingdom and waking up as the Prime Minister of the United Kingdom."

Mr Cameron reiterated his commitment to Scottish devolution but said there was a huge opportunity for the Tories to gain more of a foothold in Scotland.

He said: "I think we have a huge political opportunity here - we have now got a leader with huge stature and following in Scotland.

"We've now got a database not just of Conservative supporters but of friends of the union who look to us to defend our United Kingdom.

"We have got people who have been hardened in the battle in the streets with campaigning and we have got some targets in our sights.

"I think some of those seats, whether in the lowlands of Scotland, whether in the Highlands of Scotland, whether in Fife, whether in Argyll, whether in Aberdeenshire, whether in Perthshire, these are seats where we've got the people, we've got the message, we've got the leader, now I think we can really turn the next 200 days into an opportunity to deliver more Conservative seats in the Westminster Parliament for Scotland.

"That should be our aim, that should be our goal, and please my friends let's do everything to bring it about."

The PM thanked Labour campaign co-ordinator Douglas Alexander for his praise of the Tories' campaigning in Scotland.

Mr Cameron said: "I also want to thank someone who, and this doesn't happen very often, Douglas Alexander came up to me - hold on a minute - came up to me at the tea room of the House of Commons, you know Dougie he never uses one word where about 50,000 will do.

"But the upshot of what he said was that there was a real star at the heart of the campaign machine and that was the Tories' very own Mark McInnes."