Lancaster University is planning to charge all its postgraduate students a "top-up fee" of Pounds 30 from 1996/97.
Paul McGreal, general secretary of the student union at Lancaster, said: "This is introducing top-up fees by the back door. There is nothing to stop the university using this as a precedent to charge undergraduates a top-up fee in future years."
But Stephen Lamley, secretary of the university, said: "We have a firm policy not to introduce top-up fees of any kind."
He said that the money will cover the running costs of social provision in the university's new graduate college, which is due to open next academic year.
"The Higher Education Funding Council for England has insisted that new money be found to cover communal and social facilities," Mr Lamley said.
"We could have charged just those graduates living in student housing a joining fee - as we already do with some undergraduates - but after consultation with the student union we decided on a blanket charge."
Mr McGreal added that Lancaster receives about Pounds 1 million a year through college fees for undergraduate students directly from the Department for Education and Employment. It has been recommended that this money be transferred to the Higher Education Funding Council for England, in which case it will be rolled into the university's core funding.
"Once the university starts to lose this college fee income, they could turn to the students and charge top-up fees," said Mr McGreal.
Mr Lamley again gave a categorical assurance that this would not happen. Last academic year the university had 2,300 postgraduate students.