Lawyers, including a former European Court judge, have clashed over what Scotland's EU status might be in the event of independence.
A Holyrood committee questioned four legal figures on the Scottish government's EU proposals.
Three of the four argued that Scotland would have to apply for EU membership as a separate state.
But Sir David Edward believed membership would require "relatively small" amendments to existing treaties.
The European and External Relations committee is examining proposals for an independent Scotland's membership of the EU as set out in the Scottish government's White Paper.
The Holyrood administration has insisted it could negotiate a "seamless transition" into the EU 18 months after a yes vote in the independence referendum.
Professor Kenneth Armstrong, director of Cambridge University's Centre for European Legal Studies, told the committee that the proposed treaty amendment would be "legally implausible and incredibly politically risky".
Prof Armstrong said: "Article 49 (on accession) is the specific legal basis for dealing with an entity acquiring the status of being a member state of the EU.
"Article 48 (on treaty amendment) is to me legally implausible as it is a way of renegotiating the treaties between existing member states, and not with some other non-member state."
Patrick Layden QC of the Scottish Law Commission said that the UN accepted Russia as the successor state when the Soviet Union broke up and the EU could regard the break up of the UK in a similar fashion.
It was possible that "the UK will remain a member state but Scotland will be outside the [European] union," he said.
Pointing to interim or transitional arrangements, Aidan O'Neill QC, an expert in EU law, said the EU would not wish to deprive Scots of EU rights or deny protection to EU citizens living in Scotland. He said this would be a "nightmare scenario".
He added: "Something will be worked out, it always is."
EU figures including European Commission president Jose Manuel Barroso and European Council president Herman Van Rompuy have suggested Scotland would have to rejoin as a new state.
The UK government has said a post independent Scotland could face tough membership negotiations.
However, Sir David said: "I was judge of the European Court for 14 years. I remember repeated occasions where politicians have asserted positions which the court has found to be wrong.
"There is a gap between the vote and independence, and in that period you have an obligation to negotiate a solution to the problem.
"That is ignored by Barroso, Van Rompuy and all those who talk about it."
The scrutiny of Scotland's EU future comes ahead of the referendum on Scottish independence.
On Thursday 18 September, voters will be asked the straight "yes/no" question: "Should Scotland be an independent country?"