The Prime Minister has said: "If Scotland votes for separation that vote has to be respected by the rest of the UK, and as prime minister of the UK I would have to make that happen, though it would be heartbreaking."
The Prime Minister has told voters in Scotland: "We should be clear that if you leave the UK, you leave the currency, you leave the pound."
He also said he thought the former prime minister Gordon Brown was "spot on" when he unveiled a timetable for devolution.
"Not often you see me and Gordon in absolute agreement," he added.
David Cameron has urged Scots not to vote for independence just to punish the "effing Tories".
Speaking in Edinburgh, the Prime Minister said: "Because it is an election people think it's like a general election.
"If you are fed up with the effing Tories give them a kick.
"This is not a decision about the next five years, but the next century."
David Cameron has pleaded with Scotland's voters not to "rip apart" the union as he issued a warning that independence is a "leap into the dark" from which there is no going back.
In an article for the Daily Mail, the Prime Minister writes, "Our message to the Scottish people will be simple: We want you to stay."
"Together, the United Kingdom embodies the values the world looks on with awe and envy," he wrote, highlighting achievements such as the NHS and the state pension system.
"The United Kingdom is a precious and special country. That is what is at stake. So let no-one in Scotland be in any doubt: we desperately want you to stay; we do not want this family of nations to be ripped apart."
David Cameron has been asked if the highly-unusual move to miss Prime Minister's Questions was a sign of panic in the no camp ahead of the Scottish referendum.
I really care about this issue.
I care passionately about our United Kingdom and I want to do everything I can to put the arguments in front of the people.
In the end it is for the Scottish people to decide but I want them to know that the rest of the United Kingdom, and I speak as Prime Minister, want them to stay.
All those steps we can take, making sure people in Scotland know that they can have the best of both worlds - more powers to govern themselves but also being inside the United Kingdom.
The Government are not making contingency plans for the possibility of a Yes vote to Scottish independence, David Cameron's spokesman told Reuters.
The Queen is apparently concerned that she might be the monarch presiding over the breaking up of the Union and be the last Queen of Scotland.
A source quoted in The Mirror said: "The Queen is a unionist, there is now a great deal of concern.
“If there is a Yes vote that puts us into uncharted territory constitutionally . Nothing is certain and her being Queen of Scotland is not a given.”
Maintaining the Union is believed to have been the main topic of conversation between the Queen and David Cameron as the Prime Minister visited Balmoral over the weekend.
Palace aides have stressed that the Queen remains neutral on the issue as with all political debates.
The Prime Minister has pledged 3,500 British troops to take part in exercises in eastern Europe in 2015 and 2016.