Phil Shiner, of Public Interest Lawyers, believes charging non-Scottish students more could break the European Convention on Human Rights and also the British Equality Act.
Currently, Scottish undergraduates pay no fees, while students from the rest of the UK pay between £1,820 and £2,895 a year.
From 2012-13, students from the rest of the UK could pay up to £9,000 a year for a degree from a Scottish university while Scots will continue to pay nothing.
EU students will also pay nothing, because EU law means that they must be treated in the same way as home students.
Mr Shiner said that the UK fees system was “deeply discriminatory”. “This goes to the heart of everything I hold dear,” he said.
He is representing two students from London who have been granted a judicial review to challenge the increase in English tuition fees, because it could discriminate against poorer and ethnic minority students.
Mr Shiner has now moved his attention to the discrepancy between fees for Scottish students and those in the rest of the UK.
Holyrood, which has stuck to its position of providing free tuition for Scottish students, has raised fees for rest of UK students in order to prevent a flood of “fee refugees” who want to avoid English universities that are raising fees to up to £9,000 a year.
The Scottish government argues that its proposals, and the current system, are lawful because the differences in fees are based on domicile, rather than nationality.
“In an ideal world, no students would pay fees. Our main priority has to be to protect opportunities for Scottish students to study at Scottish institutions by maintaining free education north of the border,” a spokeswoman for the Scottish government said.
However, Mr Shiner maintains that Scottish ministers have “misinterpreted the law”.