The Conservative Cabinet minister said the Nationalists’ assertion was “at odds with the EU’s own rules of membership” and challenged the First Minister to reveal his fall-back plan in the event of delay or rejection. In a letter to the First Minister ahead of the speech Mr Salmond is due to deliver to the College of Europe in Bruges, he wrote: “Scotland’s negotiations to join the EU are likely to be complex and long and the outcome would certainly prove less advantageous than the status quo. People in Scotland deserve to have the available facts ahead of making one of the most important political decisions in the history of our union.
“The terms of EU membership which your government has said it will seek to secure for an independent Scotland are at odds with the EU’s own rules of membership.”
Mr Hague’s intervention follows an earlier claim by European Commission president Jose Manuel Barroso that Scotland may find it “extremely difficult, if not impossible” to join the EU with opposition from existing member states.
Mr Salmond hit back by accusing Mr Hague of promoting a “depressing negativity” in the referendum and claimed that the only threat to Scotland’s EU membership was “the anti- European streak” within the Conservative Party.
Mr Hague wrote that it is “far from certain” that Scotland would be a full member of the EU 18 months after the referendum and called on Mr Salmond to set out an alternative plan if the application was delayed or blocked.
The Scottish Government has proposed using Article 48 as a “suitable legal route” to facilitate an independent Scotland’s membership. Mr Hague said such a move would require all 28 member states to agree to this, and called on Mr Salmond to provide information on what guarantees he has received from member states regarding this.
The Foreign Secretary also questioned the Scottish Government’s plans for negotiating on the UK’s rebate and opt-outs like the euro, and how it would deal with pension liabilities.
Mr Hague added: “Scotland benefits from the UK’s strong voice in Europe and the UK has a proven track record in delivering for Scottish interests in the EU. The co-operation pointed to by your government in its white paper as the basis of the way forward is no substitute for the Union.
“The people of Scotland would be swapping the guaranteed negotiating power of one of the EU’s most powerful states for the hope of goodwill from 28 others – and with a much higher price tag – a poor substitute indeed.”
In response, Mr Salmond wrote to Mr Hague to insist that an independent Scotland’s EU membership application would not face opposition from other nations. He said: “The purpose of the address is to articulate the constructive role an independent Scotland could play in the European Union.
“This contrasts, of course, with the renegotiation and in-out referendum favoured by your party, leading to the inevitable conclusion that the real threat to Scotland’s position comes from the anti-European streak which now dominates your approach to politics.”
He went on to draw a direct comparison between the Foreign Secretary’s hostility to an independent Scotland’s EU membership and Tory opposition to Scottish devolution when Mr Hague led the Conservative Party in the late 1990s.
He said: “I have read your January paper on the EU. Since its publication support for independence has risen steadily in the polls, while support for your position and your party in the European elections has plummeted. As you will remember the general feeling was that your forecasts of the outcome of Scottish independence would be as wide of the mark as your now admitted mistakes about devolution when you were Conservative Party leader.
“This could be because people in Scotland can see the advantages of Scotland’s interests being promoted on Europe by a government which is clear about our position, as opposed to a London-based party still riven with division 25 years after Lady Thatcher’s address and who are prepared to put that position at substantial risk.
“In this as in much other in this debate, a positive vision is proving far more powerful than the depressing negativity of the London approach.”
In today’s speech, Mr Salmond will tell listeners that Scotland has a key role to play when it comes to energy security.
Mr Salmond will 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.
“One of the great issues facing Europe is the question of energy security. In this area Scotland is blessed. We have a key role to play in providing energy security for Europe, and in developing the low carbon technologies the world will need for the future.”
Meanwhile, Danny Alexander, UK Chief Secretary to the Treasury, is tonight expected to use a speech in London to warn that EU withdrawal would be harmful to British business.
The Lib Dem Cabinet minister, launching research into possible consequences of UK withdrawal from the EU, will say: “This is yet more powerful evidence that the UK pulling out of the EU is the very last thing our country needs.
“It will kill our hard-earned recovery stone dead. All the progress that has been built on the hard work of millions of people and hundreds of thousands of British companies would be squandered.”