David Cameron asked Gordon Brown to vet his final keynote speech in ­Scotland in the final days of the independence referendum campaign.

The request by the Conservative Prime Minister to his Labour predecessor came as the contest was on a knife-edge with polls suggesting a gap of only four percentage points between the No and Yes camps.

Mr Cameron told Scottish voters in the highly emotional speech in Aberdeen on September 16 that with his "head, heart and soul" he did not want to see them leave the UK.

Mr Brown was playing an increasingly powerful role in the pro-Union campaign, with a powerful speech the day before polling in Glasgow seen as crucial in helping to win the referendum.

Aberdeen was Mr ­Cameron's last stop in ­Scotland as the campaign reached a climax.

In the speech, Tory leader said: "I speak for millions of people across England, Wales and Northern Ireland - and many in Scotland, too, who would be utterly heartbroken by the break-up of the United Kingdom. Utterly heartbroken to wake up on Friday morning to the end of the country we love."

He also accepted his unpopularity in the campaign, saying "if you don't like me, I won't be here forever," and warned that independence was permanent.

Sources close to the Prime Minister stressed how, given the proximity of the poll, the UK Government was mindful it did not want any word out of place that could then be seized on by the Yes camp.

One Downing Street insider quipped it was the "best conversation" Mr Cameron had ever had with his Labour predecessor.