EUROPE has come to the fore in the referendum debate after academics raised serious questions over the SNP's timescale to declare independence after a Yes vote and the terms of the country's future membership of the EU.

The planned 18-month timescale to complete the necessary negotiations was described as "risible" and "wishful thinking".

Professor Adam Tomkins, an expert in public law at Glasgow University, was asked what options would be available if no agreement was reached between Westminster and Edinburgh by Alex Salmond's declared independence day of March 24, 2016.

He replied: "Unilateral declaration of independence? I don't know. Chaos."

He said the government's "proposal to move as quickly as that is risible. It's a preposterous timetable; unpicking a 307-year-old union will be a mighty ­difficult task."

He added: "It's completely unrealistic and appears to me to have been set for purely party political reasons because they want it to occur while they still have a majority in Holyrood."

The academics, appearing before the Commons Scottish Affairs Committee, also challenged the government's proposals for a seamless continuation within the European Union with all of the UK's existing treaties and opt-outs.

Professor Ken Armstrong, a ­specialist in European law at Cambridge University, said Scotland would be a new entity in the eyes of the EU.

While he conceded it was not wholly implausible that negotiations with Brussels could be completed within 18 months, Mr Armstrong pointed out how ratification of the treaty might take longer and result in "a gap between independence and accession".

He added: "The idea that all of this can be completed within that period is certainly wishful thinking."

Iain McLean, professor of politics at Oxford University, suggested that if a Labour-led UK Government were elected in May 2015 it could claim parliamentary sovereignty to reject any already agreed negotiations and "start from scratch".

Today, another expert is expected to raise doubts about the independence timetable when he appears before ­Holyrood's European and External Relations Committee.

David Crawley, a former civil servant with experience of European negotiations, said it was "largely accepted" Scotland would be welcomed into the EU eventually but that the 18-month timetable "seems unrealistic".

A Scottish Government spokesman pointed out Sir David Edward, a former British judge at the European Court of Justice, had said there would be an "obligation on all member states to negotiate terms for Scotland's continued membership of the EU between a vote for independence and Scotland becoming independent".

He also noted the UK ­Government's legal adviser, ­Professor James Crawford, a law expert from Cambridge University, had described the Scottish Government's 18-month timetable as realistic.

The spokesman added: "It is in no-one's interests to exclude Scotland from the European Union.

"As we have seen with the Treasury's sensible decision to acknowledge the Scottish Government's position on debt, when it is in the mutual interest of both Scotland and the rest of the UK, common sense prevails."

Last night in a keynote speech Nicola Sturgeon insisted it was time for those against independence "to get real" on the issue of EU membership.

The Deputy First Minister declared: "It is time to stop pretending the European Union is something it isn't. The EU is not in the business of throwing out its citizens, of ignoring democratic processes, of reducing co-operation and cutting the size of the EU.

"It is engaged in a process of ­enlargement, not contraction. It is founded on the principles of democracy and mutual co-operation among countries and citizens who share its objectives."

However, Scottish Secretary Alistair Carmichael said an independent Scotland would not retain any part of the UK's multibillion-pound annual rebate from Brussels. He also claimed Spain would veto an independent Scotland's EU membership unless its fishermen got access to ­Scottish waters.

He said: "Spain has long wanted access to North Sea fishing rights for its fleet as part of the Common Fisheries Policy because there are provisions within the CFP, which favour the UK fleet in the North Sea. I cannot think Spain would be very keen to offer Scottish fishermen the deal that they get as being part of the United Kingdom."

But a spokesman for Richard Lochhead, the Scottish Rural Affairs Secretary, urged Mr ­Carmichael to check his facts, stressing how 'relative stability' was enshrined in the CFP and applied consistently to all member states.

Alistair Darling will today contrast what he calls the security offered by ­Scotland in the UK with the risk created by what he will claim is the SNP's failure to answer questions about independence.

l Mark Carney, the Governor of the Bank of England, is expected to meet Mr Salmond and deliver a speech in Scotland later this month.