INSIGHT-United Kingdom's fate hinges on a tale of two referendums
* Scotland will hold an independence vote on Sept. 18
* Britain could vote on whether to leave the EU in 2017
* Both votes have potential to radically change the UK
* Debates on Scotland and Europe run in parallel
By Andrew Osborn and Guy Faulconbridge
LONDON, Feb 19 (Reuters) - The United Kingdom has existed for more than three centuries, and its territorial integrity has buckled only once - when Irish nationalists created a breakaway republic almost 100 years ago.
But an independence vote in Scotland in September and a possible referendum on whether to leave the European Union in 2017 mean the world's sixth largest economy now faces two consecutive threats, promising at the very least short-term volatility and, in the view of opponents of an EU exit, serious long-term damage.
If - and it is a fairly big if - Scotland votes for independence, the United Kingdom will unravel. But the "date with destiny", as Scottish nationalists like to call it, is far from a purely domestic affair.
If the break-up happens, it may affect whether the rest of the British state votes to leave the EU. Some argue that, without the relatively europhile Scots, 'Brexit' is more likely.
On the line is Britain's $2.5-trillion economy, London's position as the only financial capital to rival New York, North Sea oil, the future of Britain's nuclear deterrent, and a permanent seat on the United Nations Security Council.
The questions being asked in the two votes are different but related and the debates could influence one another and in turn shape the future of the UK.
"It is quite extraordinary. This is happening in a country which was long regarded, and regarded itself, as a beacon of constitutional stability," said Alan Trench, professor of politics at the University of Ulster in Northern Ireland.
"So suddenly to have moved from stasis into a sequence of very febrile and interlocking debates is remarkable and may say something about just how unstable that apparent stability really was."
The formal name for the sovereign state that includes England, Scotland, Wales and Northern Ireland is the United Kingdom of Great Britain and Northern Ireland. The voluntary union of England, which had subsumed Wales, with Scotland in 1707 created the Kingdom of Great Britain. It became the United Kingdom in 1800 after a formal union with subject Ireland.
The UK traces its roots as a state to the establishment of Scottish and English kingdoms in the 9th and 10th centuries and has been a member of what is today the EU since 1973.
Britons will vote whether to leave the EU in 2017 - provided Conservative Prime Minister David Cameron, who promised the vote, is re-elected next year. He is behind in the polls.
A DISUNITED KINGDOM
Converse attitudes to the two votes in different places underline how disunited the UK has become.
Polls show around a third of Scots want to break away from the UK, but that a majority of those in the UK's other three countries would like Scotland to stay. They, however, have no vote in the Scottish referendum.
Opinion is divided on the other big question too - whether Britain should remain in the EU. Most polls show about half of Britons favour leaving the bloc.
But they show that levels of euroscepticism vary sharply across the UK.
A YouGov poll in December showed that 48 percent of people in England (outside London) wanted to leave the EU; the same poll showed only 34 percent of Scots were keen to exit.
Indeed, the pro-independence Scottish National Party (SNP), is trying to persuade Scots to break away on the grounds that their country's EU future would be more secure outside the UK.
That is disputed - the president of the European Commission said this week that an independent Scotland would have to reapply to join the bloc and that membership would be "extremely difficult, if not impossible".
The Scottish and EU votes share parallels. Both are rooted in nationalism and driven by a growing sense that democracy is being subverted. Both those who want Scotland to leave the UK and those who want Britain to leave the EU think power should be exercised at a more local level.
"They are parallel debates," Rory Stewart, a Conservative lawmaker from Cameron's party, told Reuters. "We are asking the same question here. What sort of size do you want your identity to be? 500 million (the EU), 5 million (Scotland) or 63 million? (the UK)"
Cameron, an Englishman, will steer the UK through the Scottish vote, and, if he wins next year's parliamentary election, an EU referendum.
He is campaigning for Scotland to stay in the UK and has pledged to fight heart and soul to keep Britain in the EU - if he can successfully renegotiate his country's EU ties beforehand, an unknown.
His political future is tied to both votes.
SCOTLAND AND 'BREXIT'
If, as polls indicate, Scotland votes against independence on Sept. 18, most believe a British exit from the EU would be less likely.
Cameron's Conservatives are unpopular in Scotland. They have one of its 59 seats in the UK parliament, against the opposition Labour party's 41. So if Scotland remains in the UK when the next election is held his party would find it harder to win.
And no other party is promising an EU referendum.
If Cameron did win though and hold an EU referendum, the Scots could tip the balance against a 'Brexit' anyway.
Scotland may only account for around 4 million of the UK's 45 million votes, but with polls so finely balanced those votes could be decisive.
There are two schools of thought on the fallout from a Scottish yes vote. One is that it could make a 'Brexit' more likely because it would make a Conservative victory at the next election - and hence an EU referendum - more probable in terms of the new electoral arithmetic.
If Scotland voted yes it would take 18 months before it formally broke away - in March 2016 - so Scots could theoretically vote in next year's UK parliamentary election.
But anyone they voted for would have to stand down once independence became a reality and it's unlikely the next government would be formed on the basis of such votes.
The other school of thought is that Cameron, whose party regards holding the UK together as part of its historical mission, would struggle to keep his job in the event of a yes vote and that the Conservatives would be in turmoil.
Some believe a yes vote would leave Britons so bruised that they wouldn't have the stomach to leave the EU anyway.
"Scotland leaving would put the rest of the UK in a dilemma," said Stewart, the Conservative lawmaker. "It might scare people about the thought of going it alone, they might feel more isolated."
One thing there is consensus on is that if Britain did vote to leave the EU and Scotland was still part of the UK such a vote would reopen the Scottish debate, stoking Scottish nationalist demands for yet another vote on independence.
"Lots of Scots would be very unhappy about leaving the EU," said Stewart. "It would have an impact."