Prime Minister David Cameron has put the future of the centuries-old union between England and Scotland in mortal danger by stoking English nationalism, former premier Gordon Brown said on Sunday.

Brown, who led a Labour government between 2007 and 2010, told an audience in his native Scotland that Conservative leader Cameron must retreat from policies that draw dividing lines between Scotland and the rest of the United Kingdom.

Scots voted 55 per cent to 45 per cent to maintain their 308 year-old union with the England in a 2014 referendum. But since that vote support for the pro-independence Scottish National Party (SNP) has surged, helping the SNP win almost every British parliamentary seat in Scotland at May's national election.

"The union is in mortal danger unless the Tories are prepared to turn off the tap of English nationalism," Brown said at a book festival in Edinburgh, according to a copy of his speech distributed by his office.

Cameron swept to a surprise election victory in May as voters backed his manifesto based on the Conservative Party's reputation for economic competence and a promise to deliver a budget surplus.

During the Scottish referendum campaign he said it would break his heart if Scotland split from the United Kingdom.

Brown, 64, who was ranked Britain's most unpopular prime minister in half a century, emerged as an unlikely star of the referendum. He was credited with seeing off a late surge of separatist support with a series of barnstorming pro-union speeches.

On Sunday, Brown criticised Cameron's "pernicious" national election campaign, which warned voters that unless they backed the Conservative Party they would end up with a Labour-led government that was beholden to Scottish nationalists.

He said the same tactic could be used during an in/out European referendum.

Britons will vote on whether to remain a member of the European Union before the end of 2017, after Cameron has sought to renegotiate EU rules on politically sensitive issues such as immigration. The pro-EU SNP has said it would probably seek another independence referendum if Britain voted to leave.

He called for Cameron to meet in full the promises he made to Scottish voters before the independence reference to devolve powers to Scotland.

Brown also said plans to give lawmakers in English constituencies a veto over laws that only affect England, undermined Scotland and risked creating two classes of politician in parliament.

"As the government of the UK, and as a self-proclaimed Unionist party, they should instead be standing up for British values and institutions that we share in common and which connect us together," he said.