Scottish independence: Think tank report backs EU membership
No EU member state would have "a material interest" in an independent Scotland being outside the European Union, according to a new report.
The report, by the European Policy Centre think tank, also argued that the situation could cause "a legal nightmare" for other member states.
SNP MEP Alyn Smith called the report a "welcome breath of fresh air".
But the pro-Union Better Together campaign said Scotland could lose the UK's "special deals in the EU".
Mr Avery, who negotiated the UK's entry into the European Community in the 1970s, has previously spoken out in support of the Scottish government's aim of EU membership within 18 months, if Scots vote "Yes" to independence.
In its White Paper on independence, launched in November, the Scottish government said the country would look to gain membership from within the UK through Article 48 of the Treaty of the European Union.
The UK government and Better Together have argued that Scotland would have to re-apply as an independent state under Article 49 of the EU Treaty, which could take several years.
In his policy paper, Mr Avery argues that the decision on Scotland's membership would be taken by "the EU's leaders in the European Council, and they will decide on the basis of practical and political considerations".
He wrote that politically it would be "difficult to see how the Union could reject five million Scots, who are already EU citizens" but noted that some EU states, including Spain, were concerned about any encouragement to their own internal regional independence movements.
But he added: "From a practical point of view, no member state has a material interest in Scotland remaining outside the EU, even for a short time.
"This would deprive the EU of the benefits of Scotland's membership (budgetary contribution, fisheries resources, etc).
"Scotland outside the EU, and not applying EU rules, would be a legal nightmare for EU member states, whose citizens and enterprises would lose their rights in Scotland.
"No member state, particularly not the rest of the UK, would have an interest in creating such an anomaly."
Mr Smith, re-elected as one of Scotland's MEPs last week, said: "This report is a very welcome breath of fresh air, and debunks much of the nonsense peddled on Scotland and the EU by people who should know better.
He added: "This paper poses, quite rightly, a few questions over the mechanics of the process whereby Scotland will transition from a region of a member state to a member state in our own right, but the conclusions are sound."
Mr Avery's views contradict those of European Commission President Jose Manuel Barroso, who claimed in February that it would be "extremely difficult, if not impossible" for an independent Scotland to join the EU.
And a recent report by the UK Parliament's Scottish affairs committee argued that an independent Scotland would lose the UK's "special arrangements" in the EU such as the budget rebate and an opt-out from joining the Euro.
A spokesman for Better Together said: "As part of the UK we get special deals in the EU.
"What Alex Salmond needs to be honest about is what would happen to our opt-outs on the Euro and the no borders immigration scheme, as well as what would happen to our rebate.
"Telling Scots that everything will be alright on the night simply isn't credible."