Prime Minister David Cameron used his last visit to Scotland before a historic independence referendum this week to implore Scots to remain part of the United Kingdom, warning on Monday that a breakaway vote 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, pleaded with voters not to use the referendum as a protest vote, reports Reuters."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 told an audience packed with Conservative party supporters in Aberdeen, the centre of Scotland`s oil industry."Don`t think: I`m frustrated with politics right now, so I`ll walk out the door. If you don`t like me I won`t be here forever. If you don`t like this government it won`t last forever. But if you leave the UK that will be forever."Cameron`s trip was 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 harbour 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.