The Prime Minister has said he would be left broken hearted if Scotland votes to become independent.
David Cameron travelled to Perth to urge Tories to make a final push to campaign to keep the country in the United Kingdom.
Voters in Scotland are due to decide whether Scotland stays in the Union in an independence referendum on September 18.
Mr Cameron stressed that the country's future is very much "an issue of the heart".
He told a rally: "It would break my heart to see our United Kingdom break apart," and he warned that a Yes vote would be "irreversible".
He said: "If Scotland walks away from the United Kingdom and votes for separation, it's not for a trial period, it would be permanent."
The PM said Scottish First Minister Alex Salmond and the SNP had failed to make a "really positive case" for independence, accusing them of leaving vital questions about Scotland's future after a Yes vote unanswered.
"It's for those who are arguing for this radical change to come up with... justification for why this is a good idea," the Prime Minister said.
"Why is it suddenly a good idea to make your sister working in Edinburgh or your brother working in London living in a foreign country?
"Why is it suddenly a good idea to split away from the organisations that have helped this country be everything it is in our world today?
"I don't think they have made a powerful case at all and I think they have left every single key question unanswered."
Mr Cameron said there is no certainty about what currency an independent Scotland would use, if it would be a member of both the European Union and Nato, and what the costs of leaving the UK would be.
"None of the key questions have been answered," Mr Cameron insisted.
"It's extraordinary when you think how long Alex Salmond has planned this moment. He's had years to think of the answers but yet with 77 days to go you can see he is Alex the unready - not a clue about the enormous gamble he would be taking with our nation's future."
Mr Cameron said the referendum debate had featured "sound and fury" from independence supporters.
"We have had lots of arguments," he told the rally.
"We've had the noisy nationalists, it is now time to hear from the silent majority, the silent majority of Scots who do care passionately about our United Kingdom, the silent majority of Scots who don't want to take a big risk for the future of their country, themselves, their jobs, their children, their grandchildren.
"It's that silent majority we now need in the last 77 days to speak out."
He insisted people could be patriotic and at the same time vote to stay in the United Kingdom.
The Prime Minister said: "You don't have ask people to choose between the saltire and the union jack. You can be proud of your Scottishness, proud of the Scottish nation, proud of Scotland but still believe in being part of the United Kingdom.
"You can be more proud of your Scottishness than your Britishness and still believe the United Kingdom is the right future for Scotland."
Mr Cameron told the audience: "There is a patriotic No vote and in these last 77 days I need you to shout that from the rooftops.
"We shouldn't allow the nationalists to make the argument you can only be a proud Scot if you vote for separation. You can be a proud Scot and proud of our United Kingdom and vote to stay together."
He urged Scots to "rise up" and "say no to the noisy nationalists".
The Prime Minister told Tory campaigners: "Let us make sure we win it, we save our country. It's the right thing for Scotland, it's the right thing for everyone in our country."
Mr Cameron urged Scots to remember the "history of our incredible United Kingdom and all that we have achieved".
Putting his hand on his heart, he told the audience: "Yes, let's go through the arguments, the pounds, shillings and pence but let's also remember what lies in here and what we've done together."
The Conservative leader said: "We in these islands have left an indelible stamp on history."
He highlighted the UK's role in "standing up against Nazi Germany" in World War II, the BBC, the NHS and the welfare state, saying: "Those are all things we achieved as the United Kingdom."
But he stressed the case for staying in the Union was also about "what we will achieve in the future".
He said if Scots rejected independence they could have "the best of both worlds".
The Tories, as well as Labour and the Liberal Democrats, have already outlined plans to extend Holyrood's powers with more devolution if the referendum results in a No vote.
Mr Cameron said that meant Scotland could have "greater devolution" at the same time as benefiting from the "solidarity from being part of a larger organisation".
He stressed: "Voting No doesn't mean voting for no change, it means voting to stay in the United Kingdom, but to get the best of both worlds."
He also insisted the country was "safer and more secure together", arguing that in a "dangerous and difficult" world, the UK benefited from having one of the top five defence budgets, and also had police, security and intelligence services that were "respected the world over".
"If Scotland were to separate itself from the rest of the United Kingdom we lose all these networks that we can use to help keep our people safe and our country strong," he warned.
The Prime Minister went on to argue that the the UK was "stronger and more prosperous together"
He said in a competitive world "what sense does it make to start separating yourself off into a different economic unit".
Scottish Tory leader Ruth Davidson also spoke out in favour of Scotland staying in the UK, saying voting for this in September for her was "100% a vote of the heart".
Ms Davidson said: "This is our country, this is our home, this is the nation we have built. And we have done it standing shoulder to shoulder with our friends and neighbours in England, Wales and Northern Ireland.
"And every success of our United Kingdom is our success, because we built it too."