The issue of Europe and has dominated First Minister's Questions at Holyrood.
The leader of the Scottish Conservatives accused the Scottish government of wanting to "jump the queue" into the European Union.
Ruth Davidson told MSPs the former legal head of the European Council said the SNP's preferred route was "not legally possible".
First Minister Alex Salmond said even the No campaign's senior adviser supported his position on the issue.
Ms Davidson accused ministers of trying to join the EU through "a route which no other state has used" in the event of independence.
In its White Paper on independence, launched in November, the Scottish government said Scotland would look to gain membership through Article 48 of the Treaty of the European Union.
It added that countries would only use Article 49 if they were outside the EU.
Ms Davidson pressed Mr Salmond on an intervention by Jean-Claude Piris, the former director general of the legal service of the EU council.
She said Mr Piris had claimed "it would not be legally correct to try to use article 48 for the admission of Scotland as a member of the European Union."
Ms Davidson added that it was now "in black and white.. that the SNP's plan is not lawful".
She later claimed that France would need to hold a referendum to allow an independent Scotland to join the EU.
Mr Salmond argued the government's position on the issue was supported by Prof Jim Gallagher, a new adviser to the Better Together campaign.
The first minister said that Prof Gallagher had stated in a blog last year: "It seems pretty likely Scotland would be an EU member state, probably after an accelerated set of accession negotiations."
Mr Salmond told Ms Davidson: "If Prof Jim Gallagher is saying that, the guru of the Better Together campaign, can't we just accept the burden of opinion favours the position adopted by the government?"
Labour MSP Jim Macintosh also raised the issue of Europe, with a question around the legalities of an independent Scotland charging students from England, Wales and Northern Ireland to study at Scottish universities.
The question came after Jan Figel, a former deputy prime minister of Slovakia who was European Commissioner for education, said if Scotland became an independent state, students from England and Wales should receive the "the same treatment" as Scottish students, who do not have to pay fees.
EU rules prohibit states from discriminating on the grounds of nationality, meaning Scotland cannot currently charge EU students to study.
Scotland can charge students from elsewhere in the UK because the EU does regulate for discrimination within member states.
The White Paper says an independent Scotland would continue to charge students from the rest of the UK, with Ms Sturgeon later telling a Holyrood committee the policy could be justified "because of the particular circumstances of the geography of Scotland and the rest of the UK, and the policy that exists in other parts of the UK".
A spokesman for the Scottish government said the blueprint was "consistent with, and informed by" legal advice.
Academics Together, a part of the pro-UK Better Together campaign, has called on the Scottish government to publish this legal advice.
Answering Mr Macintosh's question at Holyrood, Mr Salmond said EU law allowed for "exceptional circumstances" and that ministers were still looking at proposals to charge students from elsewhere in the EU to study in an independent Scotland.
He said: "I'm aware, crucially, of the legal opinion provided to Universities Scotland which makes clear that EU law allows for objective justification where there is real evidence of exceptional circumstances and that is the position that we have outlined."