What would be the consequences of withdrawing from the EU?
BBC Radio 4's Analysis: Divorcing Europe is broadcast on Monday 16 November at 20.30 BST.
The Lisbon Treaty might not be your idea of a page-turner but there is, buried within it, a surprise: "Any member state may decide to withdraw from the Union in accordance with its own constitutional requirements."
The Treaty goes on to set out, for the first time, the process a member state would follow if it chose to leave the EU.
Polls consistently reveal a large body of opinion - sometimes up to 50% - in favour of withdrawal.
The popular and politic debate is intense. But what would actually happen if Britain did, one day, choose to leave? For Analysis Chris Bowlby set out to answer that little-discussed question.
The treaty has a hidden exit clause - but how easy would it be to use it?
Eurosceptics like Lord Pearson of Rannoch, a UKIP leadership candidate, and the Conservative MEP Daniel Hannan, point out that Britain could leave at any time, regardless of the exit process contained within the Lisbon Treaty, simply by repealing the 1972 European Communities Act.
But whether Britain repealed the 1972 Act or followed the Lisbon process, international lawyer Professor Stefan Talmon says Britain would still have to address its treaty obligations to Europe.
Britain - a net contributor to the EU - would have to extricate itself from the EU budget, deal with cooperative arrangements like the free movement of people, and negotiate complex policy areas like fisheries and farming.
Above all that, a grand deal would have be done over the future economic relationship between the UK and the EU.
The end of continental foods?
Some, like Professor Simon Hix from the London School of Economics, argue that the remaining member states would make it harder for the UK to compete in the Single Market.
Sir Stephen Wall, Tony Blair's Europe advisor, agrees that the remaining members, in the great game of national advantage, would extract a price for Britain's withdrawal.
But Daniel Hannan argues that as the EU's biggest customer, the remaining states could not afford make trade with the UK difficult. And Britain would remain a member of the European Economic Area and the World Trade Organisation, limiting the trade costs the EU could impose on a UK outside the Union.
Sir Stephen Wall, who wants Britain to remain inside the EU, concedes that in trade terms leaving would be no catastrophe.
To continue to trade with the EU, however, Britain would have to keep a large body of EU law - because all the EU's trading partners, even giants like China and the US, have to meet certain EU standards in order to trade with it.
Some, like Diana Wallis, a Liberal Democrat MEP, argue that a Britain outside the EU would find itself incorporating large chunks of EU law but with no say over the nature of those laws. The Labour MP Gisela Stuart agrees that that has been the fate or Norway.
And then there is the question of influence. Would Britain outside the EU be a diminished player in the world? Lord Pearson thinks not: we are, he says, a major power, and we would be free to re-assert our relationship with the US.
But Sir Stephen Wall argues that the US would rather Britain remained an EU member, influencing the Union from within.
It's clear that Britain could leave the EU. But like any divorce there would be lengthy negotiation.
The process would be unpredictable and the final deal would depend in part on the mood of that negotiation - whether it is acrimonious or amicable. Analysis concludes, however, that the idea that all this is simply inconceivable or irrelevant is no longer credible.
about the consequences of leaving the EU.
Gisela Stuart MP
Marco Incerti, Centre for European Policy Studies
Professor Simon Hix, London School of Economics
Lord Pearson of Rannoch, UKIP
Professor Stefan Talmon, Oxford University
Sir Stephen Wall, former Europe advisor to Tony Blair
Daniel Hannan MEP
Diana Wallis MEP
This is the last programme in the current series. Analysis will return on Monday 25th January, 2010.