By Guy Faulconbridge and Andrew Osborn
ABERDEEN/LONDON (Reuters) - British Prime Minister David Cameron is expected to appeal to Scots' emotions on his last visit to Scotland before this week's historic referendum by warning them on Monday that a vote to leave the United Kingdom would be irreversible.
With opinion polls suggesting the referendum remains too close to call, Cameron, the leader of the ruling Conservative party, which draws most of its support from England, will plead with voters not to use the referendum as a protest vote.
"There's no going back from this. No re-run. If Scotland votes "yes" the UK will split and we will go our separate ways forever," he will say, according to advance extracts of his speech given to media by his advisers.
Cameron's trip is a last-ditch effort to try to persuade Scotland's many undecided voters to reject independence. Up to 500,000 people out of more than 4 million registered voters are estimated to be unsure how they will vote.
Campaigning in Scotland is fraught with difficulty for Cameron, whose right-leaning party is unpopular with Scots who have traditionally voted for the left-leaning opposition Labour party and harbor bitter memories of former Conservative prime minister Margaret Thatcher's 1979-1990 stint in power.
Cameron's Conservatives have only one of 59 British parliamentary seats in Scotland, and the pro-independence Scottish National Party (SNP) has elbowed Labour aside in recent years to emerge as the dominant political force.
It is also in charge of the devolved government there.
Cameron has conceded his public image as a privileged Englishman with aristocratic roots does not make him the best person to advocate against Scottish independence.
Scottish nationalists criticized him for staying away in the early months leading up to the vote for being complacent, and now that he is showing his face, portray him as a condescending Englishman in no position to advise Scots on how to vote.
Details of his visits north of the English border are not revealed until the last minute for security reasons and critics say his advisers try to minimize his contact with the public to try to avoid nationalist heckling. The visit is expected to last only hours.
CONFIDENT PRO-INDEPENDENCE LEADER
Alex Salmond, the SNP leader, was out campaigning for independence in Edinburgh on Monday where he met business leaders who back the breakaway campaign.
He predicted Scotland would vote for independence and that the next time Cameron visited would be to discuss the details of the 5-million strong population's divorce settlement from the United Kingdom.
"The next time he comes to Scotland it will not be to love-bomb or engage in desperate last-minute scaremongering," Salmond said in a statement. "It will be to engage in serious post-referendum talks."
Independence supporters say it is time for Scotland to choose its own leaders and rule itself, free of control from London and politicians they say ignore their views and needs.
Cameron is likely to repeat the anti-independence "Better Together" campaign's core message: that by staying in the United Kingdom, Scotland can take advantage of the benefits of belonging to a larger, more influential entity while enjoying an ever-increasing measure of autonomy.
"No" campaigners say Scotland is more secure and prosperous as part of the United Kingdom and the end of the union would destroy three centuries of bonds and shared history as well as bring in economic and financial hardship.
Cameron's visit comes after David Beckham, the retired footballer, added his name to a petition of English celebrities who say they want the Scots to stay. The celebrity group, "Let's Stay Together", is organizing a public rally on Monday evening in London's Trafalgar Square.
It was the pro-independence camp's turn on Sunday night when a host of Scottish rock stars including the band Franz Ferdinand and Mogwai played a concert in Edinburgh.
Singer Amy McDonald told the audience: "People fight and die for this (independence) and all we have to do is put a little cross in a box. Scotland, you know what to do."
Opinion polls indicate the vote is hard to call.
Out of four recent polls, three showed those in favor of maintaining the union had a lead of between 2 and 8 percentage points. But an ICM poll conducted over the Internet showed supporters of independence in the lead with 54 percent and unionists on 46 percent. More than 4 million Scots as well as English and foreign residents, from the Highlands and Islands to Glasgow's gritty inner city estates, are eligible to vote.
The question on the ballot paper will ask simply: "Should Scotland be an independent country?
(Additional reporting by Alistair Smout and Angus MacSwan in Edinburgh and Sarah Young and William James in London; Editing by Sonya Hepinstall)