A veteran European negotiator is expected to back the Scottish Government's insistence that an independent Scotland would secure continuing membership of the European Union.

Graham Avery, honorary director general of the European Commission, who has been involved in European accession negotiations for nearly 50 years, is giving evidence at Holyrood's European and External Relations Committee today.

Speaking ahead of the committee, he said the remainder of the UK would "espouse vigorously" the SNP's preferred mode of entry into the EU to avoid a "legal nightmare".

It would not be the interests of the UK or any EU member state to ask Scotland to rejoin as a new state, he said.

"The scenario of an independent Scotland outside the EU and not applying EU rules would be a legal nightmare, create social and economic difficulties for EU citizens, and deprive the EU of benefits of Scotland's membership such as its budgetary contribution and fisheries resources," according to Mr Avery.

"To avoid this unwelcome outcome, one may expect the British Government to espouse vigorously the use of Article 48 in due course."

Article 48 of the Lisbon Treaty allows member states to propose an internal treaty amendment, and the SNP say this could be used to add Scotland as an independent signatory.

Mr Avery will join the growing ranks of EU experts verbally sparring over Scotland's future.

Three eminent lawyers challenged the SNP's claims at Holyrood last week, but they were dismissed by former European Court judge Sir David Edward, who said the UK would have an "obligation" to secure Scotland's continuing membership.

Most experts agree that Scotland will join the EU eventually and at least secure ongoing interim membership while the potentially lengthy accession process is concluded.

But Mr Avery said the SNP's 18-month timescale for EU membership is "realistic", and echoed the views of European lawyer Aiden O'Neill that any decision which deprives Scots of EU citizenship would be a "nightmare scenario".