A majority of voters in both Scotland and England want the countries to split.
Failing that, both think England should have a Parliament of its own deciding on English affairs without any involvement of Scottish MPs.
The poll findings demonstrate deep and potentially fatal cracks in the 300-year union of the two countries, and threaten to present Gordon Brown with a constitutional crisis if he becomes Prime Minister.
In a further blow to Mr Brown, a majority in both countries want him to call an election within a year of coming to power, to secure his own mandate from the voters.
The ICM survey of attitudes on the Union and nationality was commissioned by the Daily Mail in advance of next week's 300th anniversary of the ratification of the treaty which bound Scotland and England together.
It shows that nearly half of those polled think the arrangement is unlikely to survive more than 25 years.
Two out of three English voters want an end to the subsidies paid to Scotland, and a majority want to end the anomaly that gives Scots MPs at Westminster a say over legislation which affects only England.
The results suggest that Mr Brown's first months in office after succeeding Tony Blair later this year will be dominated by a massive constitutional headache. There will be crucial elections in Scotland in May and the Chancellor is braced for a damaging rejection in his own back yard as voters prepare to throw out Labour and turn to the nationalists.
The ICM survey finds a majority of voters in both countries want him to call an early election to secure his own mandate against David Cameron's Tories.
Pressure to go to the polls will intensify if he finds himself leading a party that not only failed to command a majority of votes in England at the last general election, but has been defeated in its Scottish heartland.
Mr Brown will also be disappointed that the poll indicates his attempts to promote the idea of Britishness have fallen on deaf ears on both sides of the border.
Just 31 per cent of people in England say they are British first, and only 15 per cent in Scotland.
And despite a decade of constitutional tinkering by Labour, the survey fails to record a significant level of enthusiasm for devolution in Scotland. Fewer than 40 per cent of Scots say it has been a good thing, while the level of approval in England is even lower.
With recent polls showing the Scottish National Party building a lead against Labour in the race for power at Holyrood, the Mail's survey confirms that pressure for Scottish independence is building inexorably.
It shows that more than half of Scots - 51 per cent - want Scotland to break away. So do 48 per cent of English respondents, again a clear majority of those who expressed an opinion one way or the other.
There is even stronger support for an English parliament, with 51 per cent backing the move in England and 58 per cent in Scotland.
And there was solid backing for England to have its own Prime Minister or First Minister - 54 per cent in England and 62 per cent in Scotland.
Among the English, 53 per cent want Scots MPs at Westminster to be barred from voting on issues that affect England only, such as health and education. A majority of Scots who expressed a view also want to see Scots MPs' voting rights restricted.
Nearly two out of three voters want Mr Brown to call an election within a year of taking over.
Monday marks 300 years since the Treaty of Union was passed by Parliament. The 300th anniversary of the Act of Union itself will be marked on May 1 - just two days before Scots go to the polls.
A Labour-led coalition has run Scotland since the first elections for the Scottish parliament in 1999. But voters have been turning away in droves and now appear ready to elect the SNP, which has pledged a referendum on independence.
Although Mr Cameron has promised to defend the Union, he has also called for reforms to address the so-called West Lothian Question - which asks why Scots MPs are able to vote on purely English matters at Westminster while English MPs no longer have a say on Scottish domestic affairs.
Asked about the future of the Union at the weekend, Mr Brown told the BBC Sunday AM programme: "I know that England is 85 per cent of the Union. And England at any point can outvote the rest of the Union.
"The reason why we had devolution was to recognise the different views and the decision-making processes in some other parts of the country. But at the end of the day this is a Union that is built around an England that is 85 per cent of the Union."
He added: "Let's not forget the strengths of the United Kingdom, and let's also not forget that a policy of English votes for English laws would in the end break up the United Kingdom, because the executive would have to owe its authority to simply the English members."
Brown: England-Scotland union under threat
Gordon Brown issued a blunt warning that the Union of England and Scotland is under threat 300 years after it was formed.
The Chancellor sets out his fears of a "dangerous drift" to separatism in an article to mark the tercentenary of the two Parliaments merging in 1707.
In the article Mr Brown defends the idea 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 by English MPs alone, now that devolution has been brought in.
"It is now time for supporters of the union to speak up," Mr Brown writes in the Daily Telegraph. The Chancellor's intervention will be seen as further evidence of his concern over being seen as "too Scottish" if, as expected, he succeeds Tony Blair later this year.
He even risks the wrath of traditional left-wingers tonight 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."
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