David Cameron has launched a defence of the UK as his Government prepares to put the "facts" about Scottish independence to the public.
While people in Scotland will make the decision in autumn next year, the implications will have impacts across England, Wales and Northern Ireland, the Prime Minister said.
"Britain is admired around the world as a source of prosperity, power and security," he said.
"Those glorious Olympics last summer reminded us just what we were capable of when we pull together: Scottish, English, Welsh, Northern Irish, all in the same boat - sometimes literally.
"If you told many people watching those Olympics around the world that we were going to erect barriers between our people, they'd probably be baffled. Put simply: Britain works. Britain works well. Why break it?"
Addressing matters of the "heart and head", Mr Cameron spoke out one day before the British Government publishes the first in a series of analysis papers about Scotland's role in the union.
It comes one week after the Scottish Government published a "road map" from the referendum next year to full statehood in early 2016.
Mr Cameron said Britain has built up "world-renowned" institutions such as the NHS, and "fought for freedom" in two world wars, leaving "unbreakable bonds".
He said: "But the case for the UK is about head as well as heart - our future as well as our past.
"I have no time for those who say there is no way Scotland could go it alone. I know first-hand the contribution Scotland and Scots make to Britain's success - so for me there's no question about whether Scotland could be an independent nation.
"The real question is whether it should - whether Scotland is stronger, safer, richer and fairer within our United Kingdom or outside it. And here, I believe, the answer is clear."
He added: "This big question is for Scotland to decide. But the answer matters to all of our United Kingdom. Scotland is better off in Britain. We're all better off together and poorer apart."
Scotland has its own government and parliament in Edinburgh as part of the UK, allowing decisions to be taken that affect daily lives.
Devolved powers include health and education, while Scotland has maintained an independent legal system.
In the "road-map" publication last week, it was suggested that negotiations between Scottish ministers and the UK Government, European Union and international organisations could be concluded by March 2016, assuming a Yes vote in autumn 2014.
Mr Cameron criticised the Scottish National Party (SNP) for discussing the final transition to independence before all the facts have been aired.
But Scotland's Deputy First Minister Nicola Sturgeon defended the move, saying: "The Electoral Commission has called on both sides of the independence debate to provide more information to the people of Scotland and to work together to discuss what will happen in the wake of the referendum.
"We have agreed with the Electoral Commission and published information about the transition to independence following a Yes vote.
"The Prime Minister's remarks suggest he is ignoring the Electoral Commission's advice - despite the previous calls of the Westminster government for the Scottish Government to follow their advice.
"And instead of spelling out a positive case, David Cameron is simply continuing with an entirely negative attack."
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