AN independent Scotland would have to make significant concessions to have a chance of completing negotiations on EU membership within the SNP's 18-month deadline, a UK cabinet minister has warned.

Scottish Secretary Alistair Carmichael suggested Scottish ministers would have to cave on crucial issues.

His comments come just days after his Scotland Office colleague, Conservative minister David Mundell, warned Alex Salmond would only be able to deliver Independence Day as promised on March 24, 2016, by making "huge concessions" in negotiations with the UK Government.

The SNP insist an independent Scotland will never be outside the EU because it will complete membership negotiations by 2016. Until then, Scotland will remain within the EU as part of the UK. But Mr Carmichael poured scorn on that ambitious timetable.

Experts have raised a number of issues they suggest could be sticking points during negotiations with other European countries. These include on the euro and the Schengen free travel area.

Asked about Mr Mundell's comments and whether he would say the same of EU negotiations, Mr Carmichael said: "I would very much agree with that. I think it is a big tactical blunder to set this timeframe.

"Anybody who thinks they can negotiate both with the UK and accession to the EU in 18 months is kidding themselves."

He also accused the SNP of plucking the 18-month figure "out of the air".

A Scottish Government spokesman said: "Scotland can complete the negotiations which will be required following a vote for independence within the 18-month period we have outlined - that is in line with widespread international precedent and is also a timescale described by the UK Government's own legal adviser, Professor James Crawford, as 'realistic'.

"There would be a mutual interest on the part of both Scotland and the rest of the UK in engaging constructively in negotiations after the referendum."