The identity of the United Kingdom is threatened by an "opportunist group of nationalists", Gordon Brown has warned.
Gordon Brown is widely expected to be the next Labour leader
The chancellor told the Fabian Society that some groups were "playing fast and loose" with the union of England, Scotland, Wales and Northern Ireland.
He said the UK was a country "built on shared values" which served as a "model for the rest of the world".
SNP leader Alex Salmond said Mr Brown was thinking only of his "self-interest as a prime minister designate".
Mr Brown, a Scot who is MP for Kirkcaldy and Cowdenbeath, is expected to take over as prime minister later this year.
Talking to former Labour MP Oona King at the start of the Labour think-tank's conference, Mr Brown stressed the importance of the country's shared values.
"It is very important to recognise that Britishness and Britain itself is not based on ethnicity and race," he said.
"It is founded on shared values that we hold in common: a commitment to liberty for all, a commitment to social responsibility shown by all, and a commitment to fairness to all."
He said there was now a dividing line in Britain which pitted "those of us who are prepared to support the shared values of the union" against "those who are prepared to play fast and loose with the union and put the whole future of the union at risk".
This year is the 300th anniversary of the Act of Union that merged the English and Scottish parliaments.
The pro-independence Scottish National Party is tipped to do well in the Scottish Parliament elections in May, with the independence debate likely to crop up often in the campaign.
SNP leader Mr Salmond said the chancellor "could not tolerate the idea of Scotland slipping out of Labour's control just at the point he wants to move into No 10".
"Revealingly, Mr Brown is unable to accept that, under his chancellorship, the Scottish economy has lagged behind both the UK and spectacularly, the small independent countries in Europe."
The Conservatives have suggested for some time now that it might be better if exclusively English laws were voted on by English MPs alone.
But Mr Brown said the idea of "English votes for English laws" would pull the union apart.
During a wide-ranging address to the Fabians, Mr Brown pledged support for the idea of raising the school-leaving age to 18.
And he told delegates that terrorism and security issues could not be solved through military means alone.
Earlier, he had written an article in the Daily Telegraph where he criticised the Conservatives for siding with the nationalists over constitutional issues.
In it he warned: "It is now time for supporters of the union to speak up, to resist any drift towards a Balkanisation of Britain and to acknowledge Great Britain for the success it has been and is."
He attacked today's Conservatives for embracing "anti-unionist positions" in collusion with nationalists - contrasting them with Lady Thatcher's determined support for the union.
Liberal Democrat leader Sir Menzies Campbell said the chancellor was right to highlight the "Faustian bargain" between the nationalists and the Tories.
"They may have different motives but their actions will jointly lead to the same conclusion - the break-up of the union," he said.
But shadow Scotland secretary David Mundell said the chancellor was "undermining Britishness with his support for English regionalisation, identity cards and the European constitution."
Mr Mundell also accused the chancellor of sticking his "head in the sand" over any modernisation of the union.
He told BBC News 24: "Gordon Brown simply sticks his head in the sand and says the only way to deal with that issue, and that question, is not to ask it."