The man expected to become British Prime this year Gordon Brown has issued a blunt warning that the Union of England and Scotland is under threat.
Currently the Chancellor of the Exchequer, Mr Brown expressed concern about a "dangerous drift" to separatism in an article to mark the tercentenary of the two parliaments merging in 1707.
Writing in the Daily Telegraph, Mr Brown defends the concept of Britishness amid signs that the Scottish National Party will perform well in forthcoming elections north of the border.
He also rejects calls for English laws to be decided only by English MPs, now Scotland has its own parliament.
An ICM opinion poll for The Sunday Telegraphin November found 59 per cent of English respondents supported full independence for Scotland while 52 per cent of Scots favoured the proposition.
The Chancellor's intervention will be seen as further evidence of his concern over being seen as "too Scottish" if, as expected, he succeeds Prime Minister Tony Blair.
"It is now time for supporters of the union to speak up," Mr Brown writes.
He even risks the wrath of traditional left-wingers by praising their totemic hate-figure, Margaret Thatcher, for her consistent support of the Union.
"The failure to defend and promote the United Kingdom is now becoming more a feature of the thinking of the Right.
"In contrast to Lady Thatcher, who rightly defended the Union and did so even when not expedient to do so, Conservative writers now embrace anti-Unionist positions, from independence to another anti-Thatcher stance: 'English votes for English laws' - itself a Trojan horse for separation.
"Regrettably, an opportunist coalition of minority Nationalists and what used to be the Conservative and unionist party is forming around a newly-fashionable but perilous orthodoxy emphasising what divides us rather than what unites."
Mr Brown insists: "Of course it is healthy to recognise the distinctiveness of each nation. But we will lose all if politicians play fast and loose with the union and abandon national purpose to a focus on what divides."
Mr Brown is likely to be questioned over his views on the union today, when he kicks off the Fabian Society's conference in central London.
The Chancellor will take part in a question and answer session at the left-of-centre think-tank's event, which also features leading government ministers such as Culture Secretary Tessa Jowell, Environment Secretary David Miliband and International Development Secretary Hilary Benn.
Fabian general secretary Sunder Katwala warned that Mr Brown would need to bring forward new ideas as Prime Minister if Labour was to secure an unprecedented fourth term in power.
Liberal Democrat Leader Menzies Campbell said the union needed to be "modernised" and there was more to be gained in common purpose than by separatism.
He said: "Gordon Brown is quite right to highlight the 'Faustian' bargain between the nationalists and the Tories in Britain. They may have different motives but their actions will jointly lead to the same conclusion, the break up of the union.
PA