Scotland would not be forced to reapply for EU membership for "practical and political" reasons, according to an honorary director general of the European Commission.
Graham Avery's intervention was seized on by pro-independence campaigners as evidence there will be a smooth transition to Scottish statehood after a Yes vote in 2014.
It is the latest twist in the row sparked by the Scottish Government's admission that no specific legal advice has been taken on the country's position in the EU in the event of independence. Opponents insist it is just as likely that Scotland would be forced to ask to rejoin and have to adopt the euro in place of the pound.
Mr Avery, also a senior adviser at the European Policy Centre in Brussels and senior member of St Antony's College, Oxford University, gave his view in written evidence to a Westminster inquiry on the foreign policy implications of Scottish independence.
"For practical and political reasons the idea of Scotland leaving the EU, and subsequently applying to join it, is not feasible," he wrote.
"From the practical point of view, it would require complicated temporary arrangements for a new relationship between the EU (including the rest of the UK) and Scotland (outside the EU), including the possibility of controls at the frontier with England. Neither the EU (including the rest of the UK) nor Scotland would have an interest in creating such an anomaly.
"From the political point of view, Scotland has been in the EU for 40 years and its people have acquired rights as European citizens. If they wish to remain in the EU, they could hardly be asked to leave and then reapply for membership in the same way as the people of a non-member country such as Turkey.
"The point can be illustrated by considering another example: if a break-up of Belgium were agreed between Wallonia and Flanders, it is inconceivable that other EU members would require 11 million people to leave the EU and then reapply for membership."
There are no clear precedents for Scottish independence in Europe and that transition would need to be in place before actual independence, he notes. This could take no more than two years from the referendum. The British budget rebate could require "difficult negotiations" between Edinburgh and London, as well as Brussels.
Blair Jenkins, chief executive of the Yes Scotland independence campaign, said: "People in Scotland should be reassured that an independent Scotland will remain a member of the European Union. Not only that but, as Mr Avery points out, there may be opportunities for an independent Scotland, with a full voice and vote in the EU, to emerge with a better deal than is currently available as a member of the UK."