Scottish independence would set the scene for a messy divorce, but staying together has its problems, too.

When Scotland's polling stations close their doors at 10pm local time on Thursday, a moment of nervous calm will settle on the lands north and south of Hadrian's Wall.

It will be the first lull since the start of a highly-charged four-month period of official campaigning in the referendum on Scottish independence. But the tranquillity will be fleeting. At that very moment, the future of the United Kingdom will be sealed, determined by the ballots cast by Scotland's 4.1 million voters.

Whatever the result, nothing in the UK will ever be the same again.

A Yes for independence would rip the UK apart, causing a potentially crippling reassessment of the national identity. In practical terms, it would set Scotland and the rest of the UK on track for a messy and no doubt acrimonious divorce. It would pose the greatest challenge to Britain's constitutional make-up since Irish independence in 1921 and inflict innumerable headaches on its administration, diplomacy and economy.

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"We are talking about the deeply embedded and closely enmeshed political and economic infrastructure of a 300-year-old state, its common monetary and fiscal framework and financial institutions, its NHS [National Health Service] and wider system of social welfare, its dense network of common regulatory agencies, its armed forces, its global diplomatic presence, and its membership of key international institutions from the EU to the UN Security Council, and from Nato to the G8 and G20," notes Neil Walker, Regius professor of public law at Edinburgh University.

The two sides would face an immediate stumbling block: deciding the scale and pace of independence.

Watch: Scottish referendum switches focus to Glasgow

Scotland's Government, headed by the Scottish National Party (SNP), wants the transfer of power and full independence by March 2016. "The UK Government and Parliament would be under no obligation to accept these demands, since authority over constitutional matters would remain with these UK bodies," says Norman Bonney, professor at Edinburgh Napier University.

A House of Lords' Select Committee, in a report earlier this year on the constitutional implications of Scottish independence, warned, "the UK Government would continue to have international and domestic responsibilities for Scotland between a Yes vote and the date of independence."

Separating Scotland from the UK would open up a vast box of issues. Just a few: could Scotland use sterling, managed by the Bank of England? Should there be border controls for Scots wishing to enter England, Wales or Northern Ireland, or restrictions on them working there, if Scotland is no longer a member of the European Union? What about dividing up assets, such as North Sea oil, and sharing the UK's debt? Who gets to keep cultural treasures? Then there are all the legal instruments - 31 treaties and 12,000 agreements - that would have to be handled, either renegotiated or laboriously re-approved and signed by the two countries. At Westminster, the political fall-out from a Yes vote would firstly hit Prime Minister David Cameron. There would be the question of whether the 59 Scottish MPs should be allowed to vote on matters concerning the Union, since they are legislators from a country that would be pulling out of it. The current Parliament of 650 MPs is due for re-election in May 2015, which means that Scottish MPs would be serving only until independence.

But any government taking office in 2015 risks being weakened by the departure of the 59, especially if it is Labour. The SNP rules the roost in the Scottish assembly, but at UK national level, Scottish voters plump overwhelmingly for Labour. A Labour government that had taken office with a majority in Westminster in 2015 could thus find itself with a minority in 2016. "There would be a constitutional crisis the like of which has never been seen," a former Conservative member of the Scottish Parliament, Brian Monteith, warns in his Conservativehome blog.

Watch: The practicalities of Scottish independence

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Yet a No vote will also cause storms. If the No campaign wins by only a tiny margin, Scottish independence will not in all probability go away - it would return again and again, like the "neverendum referendum" in Canada's French-speaking province of Quebec. Even a fat margin of victory would still be followed by arduous negotiations on furthering Scottish self-rule in the UK, as all the major parties have belatedly offered to do. The Scottish Parliament would be granted further powers to set and collect income tax, extend spending to more areas and have greater say over the welfare state, Chancellor of the Exchequer George Osborne says.

The fierce debate over Scottish independence has also stirred demands for regionalism in other parts of the UK. If Scotland can gain more control over its affairs from London, why can't Wales, or other far-flung English regions with a distinct cultural tradition - Cornwall, Cumbria or Yorkshire, for instance - follow suit?

"If Westminster politicians think that a No vote in the referendum is a final victory, then they are in for a rude surprise," warns Emran Mian, a former official at 10 Downing Street, now director of the Social Market Foundation policy think-tank in London. "The SNP will regroup after the result and the new tax powers already coming to the Scottish Parliament in 2015 - plus the promise of more - will give it the means to do that quickly."