Labour would need nearly a quarter of a million extra votes to win a majority at the next election if Scotland becomes independent, according to new calculations.

An analysis by John Curtice, Scotland’s leading pollster, shows that a vote for independence in September’s referendum would theoretically deprive Labour of more than 40 MPs, requiring the party to win an estimated 250,000 extra votes from the rest of the UK to compensate.

The Conservatives have one seat in Scotland. Although every national party says it is committed to protecting the union, the figures make clear how important keeping Scotland in the UK is to Labour’s electoral chances.

One Labour adviser said: “We can’t even contemplate what might happen to the party if Scotland went. This is nightmare territory for us.”

If Scots do vote for independence in September, it will take at least 18 months to negotiate the terms of the country’s exit from the UK, meaning Scottish voters will still vote in the 2015 UK elections. However, MPs from both major parties have told the Financial Times that it would be unacceptable, if Scotland had voted for independence, for Labour to try to form a government on the basis of a majority that relied on Scottish MPs.

Those MPs would attend Westminster to help negotiate the terms of the Scottish exit but would be expected to step down halfway through the parliament and could even be excluded from debating matters affecting the rest of the UK.

In such an event Labour would need a near 6 per cent swing from the Tories to form a government, requiring just under 250,000 additional votes compared with what is needed if the UK remained intact.

Labour has long been stronger in Scotland than the Conservatives, winning nearly 80 per cent of Scottish seats in 1997, compared to none for the Tories. Although Labour’s power north of the border faded during its term in government, it remains by far the largest Westminster party there, winning 41 of 59 Scottish seats in 2010. The results are so strongly one-sided in Scotland, in fact, that there is a significant electoral advantage to be had for the Conservatives in letting Scotland go – although few will admit it.

A separate Financial Times analysis shows the Conservatives would have won a 16-seat majority in 2010 had Scottish votes not been included, giving the party its first clear election win since 1992 and negating the need for a coalition government.

No other election since 1979 would have changed had Scotland been independent, but Labour would have been pushed below 30 per cent in 1983, when Margaret Thatcher was at the height of her power.

Former Tory frontbencher Michael Portillo wrote in the Financial Times this week that parts of the English party were “flippant” about the prospect of losing Scotland, given that it would make a Conservative majority more likely. But members of the party have so far remained disciplined in their defence of the union, led by David Cameron, who knows that losing this year’s vote would mark him down in history as the prime minister who lost the union.

Tim Bale, the Tory party historian, said: “This is so fundamental to the identity of the Conservative party – the idea that it stands for the whole of the United Kingdom.”

One senior Liberal Democrat MP told the FT: “We’ve been really impressed by how committed Number 10 and the Tories in general have been to fighting independence. Cameron has generally done exactly the right thing.”

The prospect of Scottish independence has thrown up a series of other possible constitutional quandaries for British politicians. Although government departments have been ordered not to do any work on planning for the possibility of separation, ministers are privately mulling the possible consequences with a mix of fascination and horror. One immediate question will be what happens to Scottish MPs currently in Westminster. This will weigh most heavily on Danny Alexander the Treasury chief secretary, whom the Scottish National party will try to co-opt into the negotiations for Scotland’s exit.

The idea of Mr Alexander serving as both a senior Scotland negotiator and working in the Treasury has led to some in Westminster floating the idea of erecting a “Chinese wall” in the government department – although others say he would have to leave his post altogether.

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