The Foreign Secretary has called on Scotland's First Minister to seek clarity on a series of issues related to the country's EU membership in the event of independence.
William Hague has written to Alex Salmond ahead of the First Minister's visit to Bruges where he will be speaking at the College of Europe.
Mr Hague 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."
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 a ll 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 18-month timescale for membership, and its "Plan B" if this does not happen, its plans for negotiating on the UK's rebate and opt outs such as the Euro and membership of the Schengen area, and how it will 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 cooperation 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."
Mr Salmond replied stating that "the UK Government have repeatedly refused to jointly approach the Commission with the precise legal scenario on Scottish independence".
He told Mr Hague that the purpose of his speech is to " articulate the constructive role an independent Scotland could play in the Europe 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 said.
Mr Salmond said the use of Article 48 had been "endorsed by many experts", while the 18-month timescale for negotiation was described as "' realistic' by Professor James Crawford, who you chose to present the UK Government's legal position and framework".
He added: "There is no mechanism to force countries into the Euro - a point the Prime Minister has alluded to - while there is no detectable wish from anyone outside your own ranks who see Scotland as anything other than in the Common Travel Area which encompasses these islands.
"You seem oblivious to the fact that the cross border pension issue is as much a matter for the rest of the UK as for Scotland, which is one of the many reasons why it would be approached by a transitional period, or, as widely anticipated by the incoming Commission, on a more general basis."
In today's speech, Mr Salmond will say Scotland's natural resources make it one of the "lynchpins" of the European Union.
He is expected to tell those gathered at the College of Europe in Bruges that Scotland has a key role to play when it comes to energy security within Europe.
Mr Salmond will say: "Scotland's vast natural resources and human talent make it one of the lynchpins 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."
He will also say: "Scotland has fully 25% of Europe's offshore wind and tidal potential. We have 10% of the EU's wave potential. We have 60% of the EU's oil reserves. 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. We have more top universities, per head, than any other member of the EU and our academics collaborate with partners across Europe.
"We have one of the largest national shares of Europe's total fishing grounds and 12 national fleets fish in our waters. The EU's fisheries policy would unravel without Scotland."