Salmond is accused of a 'lack of candour'

The academic – who last year served on an expert panel assembled by the First Minister to consider the impact of the UK Supreme Court on Scots law – warned of "tough choices and difficult compromises" in any future negotiations with Brussels.

However, echoing another leading expert, Graham Avery, he said it seemed "quite wrong" that as existing European citizens Scots would be removed from the EU or remain under a "significantly less favourable" deal.

Mr Walker, holder of the Regius Chair in Public Law and the Law of Nature and Nations at Edinburgh, gave his views in a blog for the Scottish Constitutional Futures Forum, a newly formed group of lawyers studying the implications of independence and the referendum.

It follows days of intense pressure on Mr Salmond to explain SNP Government claims a newly independent Scotland would automatically "continue" in EU membership on the same terms as the UK, including its opt-out from the single currency.

Mr Walker wrote: "Alex Salmond has done himself no favours with his lack of candour on the question of legal advice on the pathways to EU membership, and by the complacent assumption which seemed to lie behind this. His attitude, like that of some of his political opponents, has contributed to an atmosphere in which the question of a right to continuing or renewed membership is seen in unhelpfully black and white terms.

"Either future membership on the part of an independent Scotland is 'nae bother' – something which will happily unfold according to the nationalists' own agenda – or it is a path strewn with dangerous, unpredictable and perhaps insurmountable obstacles. Regardless of the legal niceties, the true state of affairs it is clearly neither of these extremes, but something in-between."

He added: "What is absolutely clear - is Scottish independence would require substantial renegotiation of the terms of membership of the European Union."

He warned there would be no unilateral right for Scotland to dictate terms and said many vital areas would have to be hammered out, including representation in European institutions, budgetary contributions, justice and home affairs and membership of the euro currency. Countries such as Spain may not welcome independence but Professor Walker insisted the prospect of Scots – as existing citizens of the EU – having their citizenship removed because they voted to leave the UK seemed "quite wrong".

His prediction of a "reasonable" attitude to an independent Scotland from other EU countries is more optimistic than the scenario put forward by Jim Sillars and Gordon Wilson. Earlier this week the former SNP leaders called on the Scottish Government to begin talks on joining the European Free Trade Assocation – made up of Iceland, Leichtenstein, Norway and Switzerland – in case the EU should order an independent Scotland to adopt the euro.

A spokesman for the First Minister: "Scotland is already part of the European Union and has been for 40 years – an independent Scotland will continue in EU membership, as many eminent experts have testified, and this latest contribution adds to that expert opinion.

"As Professor Walker points out, it is ridiculous to suggest Scotland and its five million-plus EU citizens would be deprived of that existing status while the EU has continued to welcome so many new members from central and eastern Europe in recent times. Oil-rich, energy-rich, resource-rich Scotland will be a full and valued partner in the EU, with a seat at the top table to look after our own vital national interests and able to negotiate its terms of membership from within."