Supporters of keeping Scotland in the United Kingdom have clawed back a tiny lead over separatists less than a week before Scots vote in an independence referendum that is balanced on a knife edge, opinion surveys showed on Friday.

A YouGov survey for The Times and Sun newspapers put Scottish support for the union at 52 per cent versus support for independence at 48 per cent, excluding those who said they did not know how they would vote.

"The 'no' campaign has moved back into the lead in Scotland’s referendum campaign," YouGov President Peter Kellner said in a commentary on the survey. "This is the first time 'no' has gained ground since early August."

The indication that support for keeping the United Kingdom intact has drawn slightly ahead in Scotland is of only meagre comfort to unionists; the broader picture painted by recent surveys is that the vote is still too close to call.

An Guardian/ICM poll published on Friday showed support for the union on 51 per cent and separatists on 49 per cent once don't knows were excluded.

That poll, based on telephone interviews conducted between Tuesday and Thursday, also showed 17 per cent of voters in the overall sample said they have yet to make up their mind.

Pollsters YouGov and TNS have shown a surge in support for independence since late August as the secessionist campaign led by Alex Salmond won over supporters of the traditionally unionist Labour party and some female voters in Scotland.

So far only one poll this year, from YouGov last weekend, has put the separatists in front. That survey, with a margin of error of plus or minus 2 to 3 per centage points, showed a 2 per centage point lead for the independence campaign.

"Although 'no' is back in front, the 'yes' campaign has held on to most of its gains since early August," Kellner said of YouGov's latest survey, which polled 1,268 people in Scotland between Tuesday and Thursday.

The erosion of the strong unionist lead has prompted investors to sell sterling, shares in companies with Scottish exposure and British government bonds on fears that the United Kingdom might break up.

In the event of a vote for independence on Sept. 18, Britain and Scotland would have to begin work on dividing up the $2.5 trillion UK economy, North Sea oil and the national debt, while Prime Minister David Cameron would face calls to resign.

Scotland says it will use the pound after independence but London has ruled out a formal currency union, while Britain will have to decide what to do about its main nuclear submarine base on the Clyde, which the nationalists don't want.-Reuters