David Cameron is taking perhaps the biggest gamble of his career today when he promises the British people an in-out referendum on the EU, if his party wins a majority in 2015.

As the polls currently stand, that is a very large if.

It is easier to understand the Prime Minister's motivation from the point of view of a marginal constituency in Middle England than from anywhere in Scotland. The Conservatives believe Ukip cost them an outright majority in 2010. By promising a referendum by the end of 2017, half-way through the next Parliament, he hopes to both neutralise the threat from UKIP and buy off the Eurosceptics in his own ranks.

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It is fairly clear why the British have fallen out of love with the European Union so dramatically, when Europe seems a constant source of bad news. Today's EU bears little relation to the Common Market that UK voters signed up to in 1975. The Prime Minister has a point when he describes democratic consent for the EU in Britain as "wafer thin", though anti-EU parties have never fared particularly well in general elections and EU membership remains fairly low on the list of voters' preoccupations.

Mr Cameron is right to say a referendum in the near future would make no sense because the EU could rapidly morph into something entirely different in the next couple of years. If so, we would not know what we were choosing to be in or out of, as he puts it. Most obviously, by 2017 a eurozone banking union could be either a reality or dead and buried.

The problem with promising a vote in four years' time is that it creates at least four years of uncertainty, especially for the business community and most particularly for the export-inclined Scottish business community. The debate about independence has perhaps added to the uncertainty. As if battling recession were not hard enough.

Moreover, Mr Cameron is making heroic assumptions about timescales. To organise a UK referendum on a renegotiated EU settlement in the first half of the next Parliament, the terms would need to be decided by May 2017. The chances of Brussels bureaucrats agreeing to a raft of repatriated powers on social and employment laws, policing and crime measures and protection for UK financial services at all, let alone on such a tight timetable, is surely fantasy. What if, fearful of opening a Pandora's box of demands from other states, they simply say no? Where does that leave him? Such is the toxic EU discourse in the UK, fed by fatuous myths about bent bananas and providing pigs with toys, it is easy to imagine Britain being bounced out of Europe almost by accident.

If, as he claims, Mr Cameron is a pro-European, he should concentrate on making a better case for staying in. What has the EU ever done for us? There is a huge list, starting with peace between once-deadly foes. There is also the matter of 57% of our trade depending on the EU. The Prime Minister is playing with fire. It is not inconceivable he could be remembered as the politician who oversaw both the break-up of the United Kingdom and the withdrawal of his country from the EU. Is that what he wants? Surely not.