Media playback is unsupported on your device Media caption David Cameron: "Centuries of history hang in the balance; a question mark hangs over the future of our United Kingdom"

David Cameron has urged the rest of the UK to tell Scottish voters to reject independence saying he could not bear to see the country "torn apart".

Pro-Union campaigners had "seven months to save the most extraordinary country in history", the PM argued.

In an emotional speech, he spoke about his Scottish ancestry and said the referendum was "personal" to him.

But SNP leader Alex Salmond branded Mr Cameron a "big feartie" for not agreeing to a debate with him.

Sinn Fein President Gerry Adams also waded into the debate, telling BBC Radio Ulster it was clear that the UK as a union of four countries was now hanging by a thread, which could be unravelled by referendums.

Mr Salmond repeated his challenge to the prime minister to go head-to-head with him on TV ahead of September's independence referendum, instead of lecturing Scottish voters in a "sermon from Mount Olympus".

But Mr Cameron, who chose to make his biggest intervention in the referendum debate so far at the Olympic Park in East London, said he planned to make his argument in Scotland too.

Analysis David Cameron's latest speech has reignited calls for the prime minster to debate Scotland's first minister Alex Salmond. It underscores Mr Cameron's problem with this September's referendum. He does not want to go down in history as the leader who presided over the break-up of the United Kingdom. People close to the prime minister say he is genuinely passionate about Scotland remaining within the UK. He cannot resist the temptation of making this case to the public. But every time he does nationalists pounce on his refusal to debate with Mr Salmond: prime minister to first minister. Supporters of independence point out that only last month Mr Cameron told the BBC: "The debate should be between people in Scotland who want to stay, and people in Scotland who want to go." An opportunity will present itself, they say, on 24 February when the UK cabinet and the Scottish cabinet are reported to be meeting in north-east Scotland within 10 miles of eachother. What better opportunity, says the Scottish government, for a face-to-face debate? It's the polite, political equivalent of saying, "Come over here and say that."

He said he had chosen the Olympic Park as a venue for his speech because he wanted to send a message to people living in the rest of the UK, adding "all 63 million of us are profoundly affected" by the referendum.

"That's why this speech is addressed not to the people of Scotland, but to the people of England, Wales and Northern Ireland. We would be deeply diminished without Scotland.

"This matters to all our futures. And everyone in the UK can have a voice in this debate."

Some people had advised him not to take part in the debate, said Mr Cameron, "but frankly, I care far too much to stay out of it. This is personal."

"My surname goes back to the West Highlands and by the way, I am as proud of my Scottish heritage as I am of my English heritage.

"The name Cameron might mean 'crooked nose' but the clan motto is 'Let us unite', and that is exactly what we in these islands have done."

The PM invoked the spirit of the Great Britain Olympic team, which won 65 medals in 2012, in his speech.

He said: "For me, the best thing about the Olympics wasn't the winning. It was the red, the white, the blue.

"It was the summer that patriotism came out of the shadows and into the sun, everyone cheering as one for Team GB."

He claimed there were "four compelling reasons" to save the Union: the economic benefits of being a bigger country, greater international clout, connections between people and the cultural impact of the UK.

Listing UK-wide institutions such as the BBC, the NHS and the armed forces and the country's place in the UN Security Council, Nato and the G8, as well as cultural icons like Sherlock Holmes, Emeli Sande and Scotch whisky, the prime minister said: "We come as a brand - a powerful brand.

"Separating Scotland out of that brand would be like separating the waters of the River Tweed and the North Sea.

"If we lost Scotland, if the UK changed, we would rip the rug from under our own reputation. The plain fact is we matter more in the world together."

Media playback is unsupported on your device Media caption First Minister Alex Salmond again calls for a debate with David Cameron ahead of the referendum on Scottish independence

In answer to a question after his speech, Mr Cameron urged people in Scotland who wanted to see further devolution to vote no in the referendum.

About four million people over the age of 16 and living in Scotland will be able to take part in the referendum, promised by the country's ruling Scottish National Party, on 18 September.

Alex Salmond took to the airwaves immediately after the prime minister's speech to accuse him of running scared.

"I just want the prime minster to come and debate with me and stop being such a big feartie," he told the BBC News channel.

The reason the two men needed to debate "the pros and cons of his argument against independence" on TV in Scotland was obvious, he claimed.

Scotland in numbers Image copyright Lucy Rodgers How do the UK and Scotland compare?

"If I want to talk about, let's say the bedroom tax, and demand to know from David Cameron why he thinks the bedroom tax is a good idea to impose on Scottish people, nobody else can explain that - they can say they're against it as well, but David Cameron would have to explain it."

A spokesman for the pro-independence Yes Scotland campaign said: "To use the Olympic Games as a political tool shows that the No campaign and its leader are running scared and running out of ideas. The positive case for No clearly doesn't exist.

"In September, we have a chance to put Scotland's future in Scotland's hands and ensure that we always get the government we vote for."

But a spokesman for the pro-Union Better Together campaign said: "Today the differences between the two campaigns became all too clear.

"The prime minister gave a speech about unity and people working together, while [the SNP] issued an excruciating statement that was dripping with the politics of division."