The Scottish Government can only guarantee its policy on university tuition fees will remain in place if the country stays in the UK, a former university vice principal has claimed.
Professor Susan Shaw, former vice principal of Strathclyde University, spoke out as the campaign group Academics Together challenged First Minister Alex Salmond to publish any legal advice he had received on the issue.
On Monday the group - part of the pro-UK Better Together campaign - will publish a report which will warn plans to continue to impose tuition fees on students from England, Wales and Northern Ireland after independence "would run into significant problems with European law".
The report will claim: "I f Scotland leaves the UK, and joins the EU as a separate state, the rest of the UK will become like any other European Union country.
"As a result, Scotland will be legally obliged to provide university education free to students from England, Wales and Northern Ireland.
"Even if the numbers of students from elsewhere in the UK stayed at the same level as today, this would represent a loss of income to Scottish universities of £150 million.
"If the numbers were to increase - and there would be a very strong incentive for young people from England to come to Scotland for a free education - the effect could be even greater."
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 keeping this 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.
Deputy First Minister Nicola Sturgeon has already told members of Holyrood's European Committee: ''We believe that can be done 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.''
But speaking on behalf of Academics Together, Ms Shaw claimed that Nationalist "as sertions on tuition fees don't stand up to scrutiny".
She demanded: "The Scottish Government should publish any legal advice on charging students in the rest of the UK tuition fees if we go it alone, if any such advice even exists."
Prof Shaw said there was a "real risk that the loss of funding from students in the rest of the UK would have negative consequences on our ability to teach Scotland's students".
She added: " The great irony of this issue is that the only way for the Scottish Government to guarantee its policy on tuition fees stays in place is for Scotland to remain in the UK.
"Our universities are thriving as part of the UK. Why would we want to give up the strength and security of the UK for the risk and uncertainty of independence?"
The report will state "many lawyers disagree" with the Scottish Government's belief that an objective justification could be made for the tuition fees policy.
It will quote Niamh Nic Shuibhne, professor of European Union Law at the University of Edinburgh, as saying that "the Scottish Government would face an extremely steep uphill battle to convince the EU institutions that it should be entitled to retain a practice involving systemic direct discrimination against one particular cohort of EU citizens".
A spokesman for Education Secretary Mike Russell said policies in the White Paper were " consistent with, and informed by, legal advice the Government has received".
He added: "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."
He said " very clear legal opinion" on the issue had been published by the universities themselves which "makes clear the possibility of continuing the current system within EU law".
Mr Russell's spokesman said: " It is entirely legitimate, of course, for people to express other views and that is a debate we welcome.
"What is a nonsense is for the No camp to ignore the substantial and credible advice provided to Universities Scotland - a body of impeccable neutrality on the question of independence.
"Instead they are trying to generate a scare story from Academics Together - every one of whom is signed up to the No Campaign and putting forward arguments against Scottish independence.
"When the universities themselves acknowledge that there is a good basis for this policy, the No camp should quit their self-styled Project Fear approach.
"Instead they should adopt the much-promised positive campaign and welcome the strength of Scotland's world-class higher education system as a key plank in our future economic success."