British Prime Minister David Cameron has warned Scottish voters that independence would be a "leap into the dark".
Britain's leaders of all three parties have headed to Scotland in an urgent bid to head off surging support for a split.
With only eight days to go before Scotland votes on whether to end the 300-year-old union, Cameron and opposition Labour leader Ed Miliband agreed to skip their weekly debate in the House of Commons and travel north of the border.
The sudden move, announced only a day earlier, reflects a new sense of urgency in the unionist camp after recent polls showed the two sides of the debate were now neck-and-neck.
"The United Kingdom is a precious and special country," Cameron wrote in Wednesday's Daily Mail newspaper.
"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.
"If the UK breaks apart, it breaks apart forever. So the choice for you is clear: a leap into the dark with a Yes vote, or a brighter future for Scotland by voting No. You can have the best of both worlds in the UK."
Liberal Democrat leader Nick Clegg, the deputy prime minister in the coalition government, will also hit the campaign trail.
Just a few weeks ago, there had been little concern in Westminster about the referendum on September 18, with polls consistently showing an easy victory for the unionists.
But a YouGov poll at the weekend set the cat among the pigeons, putting the pro-independence "Yes" camp ahead for the first time, while another on Tuesday showed the two sides were tied.
In response, all three major parties unveiled a timetable for the transfer of new powers over income tax and welfare spending to the devolved Scottish government, and said work would begin immediately after a "No" vote.
The YouGov poll on Sunday put the "Yes" camp ahead by two points, a major development given that the same polling company put them 22 points behind one month earlier.
Another poll on Tuesday by TNS Scotland confirmed the two sides were neck and neck, with respondents who said they were certain to vote evenly split at 41 per cent.
"This poll reveals a remarkable shift in voting intentions. It's too close to call," said TNS Scotland head Tom Costley.
The polls show Labour voters in particular are shifting towards independence, prompting former Labour prime minister Gordon Brown to launch a six-day tour of campaign events.
Cameron had been taking a back seat but the polls have changed the political landscape, prompting a rare show of unity with his rivals - although they are not expected to share a platform when in Scotland.
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