Britain's Prime Minister David Cameron gestures as he delivers a speech at the Aberdeen Exhibition and Conference Centre in Aberdeen, Scotland, Sept. 15, 2014. Cameron appealed 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. Reuters

British Prime Minister David Cameron made an impassioned plea to Scots to stay within the United Kingdom, comparing Scotland's historic referendum on independence to a "painful divorce." Recent polls suggest the referendum, set for Thursday, will likely be a close decision, and Cameron's visit to Scotland Monday to campaign for the "No" vote underscores Britain's growing concern over the independence movement.

"Independence would not be a trial separation, it would be a painful divorce," Cameron said in the Scottish city of Aberdeen, an important oil and gas hub for Scotland. "This is a decision that could break up our family of nations, and rip Scotland from the rest of the U.K. And we must be very clear. There's no going back from this. No re-run. This is a once-and-for-all decision. If Scotland votes yes, the U.K. will split, and we will go our separate ways forever."

With just three days until the referendum, roughly one in 10 Scots remain undecided. The referendum question is: “Should Scotland be an independent country?”

Cameron has heavily campaigned for the "no" vote in recent days after largely ignoring the referendum campaign for months, according to The Globe and Mail. "I speak for millions of people across England, Wales and Northern Ireland -- and many in Scotland, too, who would be utterly heartbroken by the breakup of the United Kingdom. Utterly heartbroken to wake up on Friday morning to the end of the country we love," he said.

First Minister Alex Salmond, who is leading the "yes" campaign, has predicted a victory for the independence effort. He has said an independent Scotland would be “a more prosperous country, and also a fairer country.”

Cameron's speech came after Queen Elizabeth, who also had declined to enter the debate, finally addressed the referendum Sunday, saying she hoped “people will think very carefully about the future.” Roughly 72 percent of people in Scotland say Cameron has handled the referendum campaign badly, a recent YouGov survey found.