Former Prime Minister Tony Blair said on Saturday that he hoped Scots would vote against independence, saying that breaking up the United Kingdom would not be a sensible step.

The fate of the United Kingdom is hanging on a knife edge as polls show Scots are evenly divided between support for the union and support for separatists led by nationalist leader Alex Salmond.

"I hope Scotland votes to stay part of the United Kingdom," Blair, who was born in Scotland, said at a security conference in the Ukrainian capital of Kiev.

"For all the reasons given by all the party leaders of the UK, in the 21st Century to rip up the alliance between our countries would not be sensible, politically, economically or even emotionally."

Blair, who served as British prime minister from 1997 to 2007, has been largely absent from the debate over Scottish independence, though his successor, former Prime Minister Gordon Brown, has taken a leading role in the unionist campaign.