Scottish independence: No 10 silent on David Cameron's future if Yes vote wins the day

Spokesman says 'it's a hypothetical issue' as to whether Prime Minister would resign over independence

Downing Street last night refused to comment on mounting speculation that David Cameron would be forced to resign if Scotland votes for independence in September.

While remaining behind in the polls, the Yes campaign has narrowed the gap over recent weeks to four points, and there are fears inside Whitehall that Scottish voters will back independence, triggering a constitutional crisis.

In a speech in Bruges tomorrow, Alex Salmond will declare that Scotland makes a significant contribution to the European Union through its "vast natural resources and human talent" which would be bolstered if the nation won independence.

But in response, Danny Alexander, the Chief Secretary to the Treasury, is expected to claim in a speech on Wednesday that the Scottish government has made uncosted policy pledges and "over-optimistic assumptions" about its oil revenues and energy reserves.

The two speeches reflect the growing bitterness in the referendum battle, and come just days after another setback for the pro-Unionists. The Confederation of British Industry was forced to reverse its decision to register with the Electoral Commission as a "non-party participant" campaigning in favour of a No vote, after a number of firms and organisations left the group in protest. There are now further signs of jitters inside government and in Better Together, the umbrella group fighting to keep the United Kingdom. Writing in The Times, the respected political commentator Matthew Parris said that under a Yes vote on 18 September: "I cannot see a picture in which the Prime Minister who conjured that referendum out of nowhere simply carries on."

Responding to the article, a spokesman for the Prime Minister said: "We don't answer hypothetical questions. There are a lot of people commentating on it who ought to get stuck into arguing for what they believe."

In his Bruges address, Scotland's First Minister is expected to say: "Scotland's vast natural resources and human talent make it one of the linchpins of the European Union. Our huge energy reserves, our economic and financial contribution, our fishing grounds, our academic, cultural and social links, and our commitment to the founding values of the European ideal place us at the very heart of the EU."

Mr Salmond will lay Scottish claim to 25 per cent of Europe's offshore wind and tidal potential and 60 per cent of the EU's oil reserves, adding: "But our importance to the European Union stretches further. As one of the wealthiest countries, Scotland is a net financial contributor to the EU and will remain so as an independent member."

Yet Mr Alexander, in his speech in Edinburgh, will say it is time to start "busting the myths around Scottish independence". In a few weeks, the Treasury will publish a "major fiscal analysis" on the benefits of the UK and the costs of independence.

Treasury officials say that there is an absence of detailed costings from Mr Salmond's government, which is hiding how economically damaging independence would be to Scotland.

Mr Alexander said: "The Scottish Government's White Paper contained lots of promises, but nothing credible to back it up.

"People are beginning to realise that they can't answer even the most basic questions. If their promises seem too good to be true, it's because they are."