The British prime minister has urged Scots to vote against independence in his most forceful speech yet, by promising Scotland sweeping new powers in the event of a "No" result, ahead of this week's historic referendum.

In his last visit to Scotland before Thursday's election, David Cameron warned on Monday that a vote for independence wouldn't be a "trial separation" but a "painful divorce".

"There is no going back from this, no re-run. This is a once-and-for-all decision," Cameron told a crowd in Aberdeen.

With opinion polls suggesting the referendum remains too close to call, Cameron, the leader of the Conservatives, which draws most of its support from England, pleaded with voters not to use the referendum as a protest vote. 

There is no going back from this, no re-run. This is a once-and-for-all decision

David Cameron, UK prime minister

"Head, heart and soul, we want you to stay," he said.

Cameron also offered to allocate additional powers to the Scottish parliament over tax, spending and some welfare services if the UK remained intact.

Cameron's trip was a last-ditch effort to try to persuade Scotland's half a million undecided voters to reject independence.

Al Jazeera's Laurence Lee, reporting from Aberdeen, said the plea could sway many of the undecided.

The Conservative party have only one of 59 British parliamentary seats in Scotland, and the pro-independence SNP has eclipsed Labour in recent years as the dominant political force.

Alex Salmond, the leader of the SNP, was also campaigning on Monday urging Scots to choose its own leaders and rule itself.

He said Scotland would vote for independence and that the next time Cameron visited, he would discuss the details of the five-million strong population's divorce settlement from the UK.

"The next time he [Cameron] 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."

Scotland votes on Thursday whether to end a 307-year-old union with England and break away from the UK.

Source: Al Jazeera and agencies