David Cameron pleaded with Scotland 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.
During a visit to Scottish Widows offices in Edinburgh, the Prime Minister said he cared "far more about his country than his party" and would be "heartbroken" if the UK was torn apart.
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Mr Cameron, Nick Clegg and Ed Miliband are all in Scotland on separate visits in a bid to persuade Scots to vote No to independence on September 18, offering instead a fast-tracked timetable for further devolution.
Mr Cameron said: "I care far more about my country than I do about my party.
"I care hugely about this extraordinary country, this United Kingdom that we've built together
"I would be heartbroken if this family of nations that we've put together was torn apart."
He told an audience: "I like to think the Better Together campaign has made some very strong arguments of the head.
"But I think it's also important we make those arguments of the heart, we talk about what we care about and how we feel about this amazing country, the United Kingdom, that we've built together."
He warned the referendum is not about kicking the Tories, and that a Yes vote is a decision about the next century.
He 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."
He continued: "This vote is not about whether Scotland is a nation or not, Scotland is a nation, it's a strong, proud nation with an extraordinary history and incredibly talented people.
"But it is a nation that has chosen over the last 300 years to be part of a family of nations. A family of nations that enables this great country to punch way above its weight in the world.
"This vote is not about Scotland versus Britain it's about two competing visions for Scotland.
"I hope you choose the vision of Scotland that is about Scottish pride, Scottish patriotism, Scottish nationhood, but is also about being part of the family of nations we have created."
Speaking on the issue of more powers in the event of a No vote, the Prime Minister pledged there would be "rapid and very comprehensive moves" to ensure Scotland has more power.
In an article for the Daily Mail ahead of his visit, the premier set out some of the United Kingdom's greatest achievements - including the Scottish enlightenment, the abolition of slavery and defeating fascism - to highlight "what is at stake" on September 18.
The rest of the world "looks on with awe and envy" at the modern British achievements such as the National Health Service and state pension system, Mr Cameron added.
He wrote: "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.
"Across England, Northern Ireland and Wales, our fear over what we stand to lose is matched only by our passion for what can be achieved if we stay together.
"If we pull together, we can keep on building a better future for our children. We can make sure our destiny matches our history, because there really will be no second chances. If the UK breaks apart, it breaks apart forever."
Mr Cameron insisted the Better Together campaign has provided clarity, while "those who support separation offer only question marks".
He claimed the Yes campaign can only offer voters blank pages when it comes explaining what money Scotland would use without the pound and what would happen over membership of the European Union as well as public funding issues.
A Yes vote would be a "lucky dip" while support for Better Together would be the "best of both worlds" and lead to a stronger, more autonomous Scotland, he said.
"Just because our countries are great together, that does not mean we cannot be even better. That is why a No vote doesn't mean a vote for the status quo - No doesn't mean no change. It means significant further devolution for Scotland - major new powers over tax, spending and welfare all being passed to Scotland.
"This is not about Scotland versus the rest of the United Kingdom. It is about two competing visions of Scotland's future."
Speaking in Cumbernauld, Ed Miliband warned that ndependence would not deliver social justice, but plunge the country into a "race to the bottom" which would harm wages and working conditions.
He also said that the case for Scotland to stay in the United Kingdom came from the "head, heart and soul".
He restated his pledge to deliver legislation for further devolution of powers on taxation, work and social security to the Scottish Parliament in the first Queen's Speech of a Labour government, insisting that the "thirst for change" expressed by voters north of the border was shared across the United Kingdom.
Rejecting Scottish National Party efforts to cast all the unionist parties as part of a Westminster elite, Mr Miliband promised change on issues like the "bedroom tax", the minimum wage and zero hours contracts if Labour removes the coalition from power in the 2015 general election.
And he stressed that he was a long-standing opponent of the policies of Margaret Thatcher which caused widespread anger in Scotland in the 1980s and sparked much of the modern demand for independence, describing how he marched against the poll tax and "hated what she was doing to the country".
"Scotland's values of fairness, justice and equality have shone through in this referendum campaign," said Mr Miliband.
"But to meet those values I know we have to change our country.
"Together we can do that. Not by irreversibly breaking apart, with all the risks that means. But by building a different future."
Mr Miliband acknowledged that he has no Scottish roots and no vote in the September 18 poll.
But he added: "I do care passionately about the outcome of this referendum. I care because of the values that motivate me as a person, the reasons I am in politics."
The Labour leader said his argument came from the "head, heart and soul".
"Head: because I believe we can better create a more equal, a more socially just society together than we can alone.
"Heart: because of the ties that bind us together and would be irreversibly broken by separation.
"And soul: because it is solidarity that built the great institutions like our National Health Service and can tackle the great injustices of our time."
Labour's offer of enhanced devolution "beats by a long way the change on offer from the other side of the argument on social justice", said Mr Miliband.
"As six Scottish trade unions set out at the weekend, independence would lead to a race to the bottom, as corporations seek competitive advantage from being in one country or another, seeking to drive down tax rates, wage rates and terms and conditions," he said.
"And don't take my word for it. Because Alex Salmond himself is proposing a 3p cut in corporation tax, taking money away from working families."
Independence would take Scotland "away from social justice, not towards it - a race to the bottom", he warned.
Mr Miliband recalled that his father Ralph, a Jewish refugee from Nazi Germany, served alongside Scots stationed in Inverkeithing in Fife during the Second World War.
And he said he wanted to be able to take his children back to Inverkeithing as part of the United Kingdom to tell them about their grandfather's service.
"It has been said that the emotional argument lies with independence," he said.
"Not for me. Not for so many people across our country. Because our hearts lie with you."
Solidarity between the nations of the UK had helped defeat fascism, build the NHS and secure a national minimum wage, said Mr Miliband.
And he said that continued solidarity meant that "the family in Edinburgh cares about the poor child in London, the working people of Liverpool care about the working people of Glasgow ... the young people of Manchester care about pensioners in Paisley".
In his most impassioned call yet to the Scots for a No vote, the Labour leader said: "Please stay with us. Stay with us because we are stronger together. Stay with us so we can change Britain together."
In Rutherglen Main Street, campaigners from both sides of the referendum debate faced off as John Prescott hit the campaign trail.
Yes supporters jostled with Better Together campaigners as Mr Prescott visited South Lanarkshire on the Scottish Labour battle bus.
He was joined by Better Together campaign leader Alistair Darling and deputy Scottish Labour leader Anas Sarwar.
Mr Prescott said he was in Scotland to talk particularly to Labour voters, and urged Scots to remain part of the UK.
He said: "I'm not here to tell you anything about the arguments about currency union and uncertainties.
"What I want to say is what Scotland has done for Britain and what Scotland and its values have brought into the essence of Britishness.
"British has come about because many of the struggles and strains that have gone on in this country started in Scotland. It's your character, your contribution to the intellectual contribution, to medicine, to education, you have always led the way, that's the reality and I say that as a Welsh person.
"Of course there's a lot to be done but the leadership has come from Scotland and I'm here as a Welshman, representing a northern constituency most of my life, to argue to keep that character, to keep that contribution that is Scottish."
He was surrounded by placard bearing "No Thanks" and pro-independence campaigners and at one point his words were almost drowned out by shouts of "Yes".
Meanwhile, speaking in Selkirk, Deputy Prime Minister Nick Clegg said there is no more momentous decision than the one facing people in Scotland next week.
He denied claims that the pro-union parties' joint offer of more powers for Scotland in the event of a No vote was a "last-minute" panic measure to shore up support for the union.
Speaking in the town's Market Square where he met local activists, Mr Clegg said: "People say this is all last minute, (William) Gladstone was campaigning for home rule in the 1880s. This is something my party has been campaigning on for generations.
"What's new, and that crystallised on August 5, was when the three main parties said we all now agree there is going to be irrevocable process of further significant and sweeping new devolution to Scotland - assuming that the people of Scotland decide that Scotland, as I fervently hope they will, remains part of the UK - in the very near future that will cover taxation, that will cover welfare and will cover borrowing, amongst other areas.
"Whatever the result on the 18th, even if Scotland votes to remain part of the UK, let's be clear, the status quo has gone, not only for Scotland but for the whole of the UK.
"In my view we will be entering into not only a very exciting new chapter of devolution of power to Scotland but also be entering into a very exciting new chapter of decentralisation and devolution throughout the rest of the UK including in England as well.
"This is a momentous event which will affect everybody north and south of the border forever and that's why it's such a significant decision."
Mr Clegg, who was heckled by local Yes supporters who said he failed to speak to them, brushed off suggestions that his presence north of the border, and that of the other two major party leaders, would galvanise support for independence in Scotland.
He said: "Alex Salmond is very good at sneering but I think at the end of the day if political leaders were not here I think quite rightly people would be asking questions about why on earth we were not putting our side of the case, because there can be no more momentous decision.
"I'm an English MP representing an English constituency and I'll be back in Sheffield tomorrow night where I can tell you everybody will be talking about this. We don't have a vote in Sheffield but people care about it passionately because they know how important it is and that is why it is right that people, even people like me who don't have a vote, nonetheless come and put our side of the story, which is that we really can have the best of both worlds.
"We can give these significant new powers to Scotland and celebrate the differences as we do already in the UK, the great differences that exist between the nations that make up the UK, but not do so by just rupturing all the links that have bound us together for so long, and which let's face it, have created in this family of nations, a union which has been remarkably successful."
Asked if his party would be willing to compromise on its position in determining which new powers should be devolved to Holyrood, Mr Clegg said: "Yes, of course the parties will need to compare their different blueprints."
He added: "We also need, and this is built into the plans that Gordon Brown announced, a very important phase where civil society, NGOs, communities, towns, villages, cities, trade unions, employers, churches also say what they would like to see in this new, exciting chapter of devolution.
"The parties have their opinions but so do people beyond politics and we need to make sure that everybody can congregate together."
Former Scottish Secretary Michael Moore, who was with Clegg in Selkirk, said the recent poll putting support for independence ahead of backing for the Union, was "re-energising" those in the No camp.
Defending the visits of the party leaders north of the border, he said: "You'll notice that the only people who say that it plays into the Yes camp's hands is the Yes camp.
"The reality is that this really helpfully focuses on the argument and the fact that, yes, the rest of the UK does care about this.
"Lots of people are realising what a massive change it would mean, not just in Scotland but to the rest of the UK too, so I welcome it.
"The fact that all three of them are here in Scotland today making their point, emphasising about more powers I think is very positive."