Jan Figel, a former deputy prime minister of Slovakia who was European Commissioner for education, training and culture between 2004 and 2009, said if Scotland left the UK and became a member of the European Union (EU), students from England and Wales should receive the "the same treatment" as Scottish students - who do not have to pay to study at universities north of the border.
Current EU rules prohibit states from discriminating on the grounds of nationality, meaning Scotland has to give fee-free university education to EU students from outside the UK in order to keep studying at university free for Scottish students.
But it can impose tuition fees on students from England, Wales and Northern Ireland because the EU does regulate for discrimination within member states.
The Scottish Government's white paper on independence proposes to continue to charge students from England, Wales and Northern Ireland if Scotland votes to leave the UK in next year's referendum.
Under European laws, Scottish ministers would need to make an ''objective justification'' for doing this.
The Scotland's Future white paper argues that the "unique and exceptional circumstances" an independent Scotland would face as a result of factors such as the "relative size of the rest of the UK" and the policy of university tuition fees south of the border would allow the Scottish Government to "justify objectively the continuation of our current policy" in a way which is "compatible with EU requirements".
But when asked if the proposal to continue to charge students from the rest of the UK to study at Scottish universities was legal, Mr Figel told The Daily Telegraph: "This would be illegal, this would be a breach of the treaty."
He said: "If Scotland is an EU member state, from that day on it must apply the non-discriminatory rule which is linked to the free movement of persons.
"It must apply the same treatment for English and Welsh citizens as it does for Scottish (citizens)."
It comes after a spokesman for European Commissioner for Education and Youth Androulla Vassiliou was reported to have said: "Unequal treatment based on nationality (or on residence, which in many cases is de facto based on nationality) is regarded as discrimination which is prohibited by Article 18 of the Treaty on the Functioning of the EU, whenever such treatment falls within the scope of Treaty. This is the case for the conditions of access to education, including tuition fees."
Deputy First Minister Nicola Sturgeon has already told members of Holyrood's European Committee that an objective justification can be made for the policy "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 Scottish Government spokesman said: "The white paper is consistent with, and informed by, legal advice the Government has received. The content of any legal advice is confidential.
"By long-standing convention, successive Scottish and Westminster governments have not disclosed the source or content of legal advice other than in the most exceptional circumstances.
"The requirements of the EU allow for objective justification - that is clear evidence of exceptional circumstances. This is explicitly acknowledged on page 199 of Scotland's Future, where it expressly identifies objective justification as the basis of our approach. This is a point made by Universities Scotland too.
"Our policy is based on the unique and exceptional position of Scotland in relation to other parts of the UK, on the relative size of the rest of the UK, on the fee differential, on our shared land border and common language, on the qualification structure, on the quality of our university sector and on the high demand for places.
"We believe that these distinctive characteristics will enable us to justify objectively the continuation of our current policy in a way which is consistent with the principles of free movement across the EU as a whole, and which is compatible with EU requirements."
Academics Together, an arm of the pro-UK Better Together campaign, has called on the Scottish Government to publish its own legal advice to support its tuition fee claims.
David Caldwell, former director of Universities Scotland and a member of Academics Together, said: "Experts in European law have made clear that the SNP's plan to charge fees to students from elsewhere in the UK in an independent Scotland would breach EU law.
"Even the opinion obtained by Universities Scotland, which is the source on which the White Paper seems to rely, concedes that there is no guarantee the SNP's argument would stand up in a European court.
"Earlier this week I wrote to the First Minister urging him to publish any legal advice he has on this issue.
"Since there is such overwhelming legal opinion that the SNP plan would not be legal, it is vital that, if the SNP Government has any legal advice indicating differently, it must publish it."
Speaking at First Minister's Questions, Mr Salmond said: "Academics Together are campaigning for a No vote, so perhaps these views are of no great surprise.
"But I'm also aware, crucially, of the legal opinion provided to Universities Scotland, a body we can agree is of impeccable neutrality on the concept of independence, 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."
He confirmed that ministers are still looking at proposals to charge students from elsewhere in EU tuition fees to study in Scotland.
"We're still looking at the policy, but of course the important aspect that we recognise is that the policy that we are outlining in the White Paper, of course supported by the legal advice to Universities Scotland, of objective justification seems to us to set out a route whereby we can retain free education in Scotland," Mr Salmond added.