senior Tories fear a vote for Scottish independence in an autumn 2014 referendum could deprive the Conservatives of an overall majority or secure Labour a narrow victory at Westminster because there would not be enough time to get rid of Scottish MPs from the House of Commons.

The Prime Minister is coming under pressure from his party colleagues to secure a poll next year, because they believe that if Scots voted in autumn 2014 to leave the Union, there would not be enough time to pass a constitutional bill to decouple Scotland before the next General Election campaign, which would begin in March 2015.

There are also fears Labour, and Liberal Democrat backbenchers, could deliberately try to delay the bill.

If they succeeded, at the next Westminster election in 2015, there would still be 50 Scottish MPs. On past form, most would be Labour. If the poll was close, this could either deprive the Tories of an overall majority or allow Labour to sneak a win.

"It would be extremely messy," one senior Conservative source close to Mr Cameron told The Herald. He said a second General Election would almost certainly have to be held, because the constitutional bill getting rid of Scottish MPs would be passed within about a year of the new Westminster Parliament sitting.

Another Conservative source said: "This would not be a straightforward business. The PM believes there would be very little time to push through a major constitutional bill. That's one of the main reasons why autumn 2014 is not feasible. "

Scottish Secretary Michael Moore last week announced the UK Government's preferred date for the independence poll was September 2013, contradicting Mr Salmond's assertion that there was consensus on autumn 2014.

The new Scottish Sun on Sunday said yesterday that the referendum would take place on Saturday, October 18, 2014, but The Herald understands the Scottish Government has only narrowed down the date to a three-week window.

Mr Salmond yesterday insisted during a phone-in to Northsound radio in Aberdeen that no date had yet been fixed.

He said: "We are only a month into a three-month consultation. I think at the last check there were 2700 responses already, so we are heading perhaps towards up to 10,000 responses to the consultation. Once all of these are analysed we will announce what the date will be, and not before then."

It is by no means certain that a Saturday will be chosen, with a traditional Thursday poll still a possibility, giving six possible dates in October.

Meanwhile, Scottish Tory leader Ruth Davidson has been put on the back foot by the prospect of members of her parliamentary group joining the new Devo Plus campaign group.

The new "cross-party and non-party" group will launch in Edinburgh tomorrow to look for a model for greatly enhanced devolution short of full tax-and-spend powers.

One Tory MSP will be unveiled at the meeting, which will represent a challenge to the approach of Ms Davidson, which has been to limit further reform to the current Scotland Bill.

Former MSP Ted Brocklebank also launched a newspaper broadside against Ms Davidson yesterday, saying David Cameron's offer of more powers if Scots reject independence had "destroyed any remaining shreds of her credibility."