This term, evidence has emerged that officials are making it harder for overseas PhD students to extend their visas. The Home Office assumes that a PhD is a three-year course, and any extension beyond that period is discretionary.
Three years, however, is very much an ideal completion rate, and one that many academics regard as the minimum time needed to finish a doctorate. Theresearch councils, which fund most home doctoral students, do want to see PhDs completed more quickly. But even they regard three years as optimistic.
In assessing the performance of university departments in postgraduate training the research councils measure the number of students who complete within four years. In 1993, the target for the Economic and Social Research Council (ESRC) - one of the toughest - was 73 per cent. Across the academic disciplines a substantial minority of postgraduates will take five years or more.
Until now, the immigration authorities appear to have accepted this at face value. But in the last few months, student unions have dealt with far more than their normal visa caseloads. Postgraduates who have been in the country for three years are finding it harder to gain permission to stay to continue their research, or to write up their theses, although the Home Office denies that the rules have changed.
Writing up poses particular difficulties. Students may no longer be using lab space or receiving active supervision. In some universities, a doctoral candidate who has finished his or her research but not yet finished the thesis is granted a "continuation period" at a reduced fee - a sort of academic limbo with which immigration officers may not be familiar.
The problem is particularly acute at Oxford and Cambridge. In part, this may be because of the collegiate structure, which means the policy on writing up is less consistent than at other institutions where postgraduate issues are co-ordinated centrally.
At Cambridge, the graduate union reports overseas students receiving letters, reminding them that they are expected to leave the country by a certain date. This is usually after their three years have expired.
"In light of the Asylum and Immigration Bill going through Parliament, the Home Office is tightening up on existing powers," believes Chris Horscroft, president of the graduate union. He advises affected students to lodge an appeal with the Home Office. An appeal often takes as long as a year, and this can give a student enough time to write up and submit his or her research. This is not ideal, because it could make it harder for that student to gain entry to the UK in the future, but it is a workable solution, union officers believe.
"Most of the students only need a couple of months," explains Donna McLaughlin, vice-president for welfare at Oxford University students' union. "They will be out of the area before the appeal is heard."
Ms McLaughlin also believes the Government's desire to clamp down on immigration is at the root of the problems. But, in the absence of more definite advice from the Home Office, the advice she can give students is limited.
There have only been about a dozen cases this year - although not all affected postgraduates will have contacted their students' unions - but universities are taking the issue seriously.
Overseas students bring an additional academic stimulus to UK campuses, and contribute a substantial sum of money to this country's coffers. An European Union student will pay around pounds 2,500 a year for a PhD place; overseas graduates may pay three times as much.
In fact, education is regarded as such an important export for the UK that it is included in Department of Trade and Industry initiatives. A working party of vice-chancellors and senior staff, tours high-growth regions, such as south-east Asia and China, to persuade graduates to study here rather than elsewhere in Europe, the United States or Australia.
The Committee of Vice-Chancellors and Principals has identified the value of overseas students to the UK economy to be around pounds 1bn annually.
International officer, Roger Blows, agrees that there would be a serious problem should the practice of refusing extensions to postgraduate students become widespread. "If anything, completion rates have improved," he says. "But it is very difficult to do a PhD to a target date or to a timetable."
Mr Blows is particularly concerned that overseas students are given the full picture on the immigration situation when they apply. Many overseas graduates were annoyed by the Home Office's recent decision to insist that anyone in this country on a student visa had to return home to apply for a job. This can be extremely inconvenient and has led to good graduates being lost to UK universities.
"We are talking about students who do not cost the UK anything, and actually add to revenue, but these are difficult points to get across," Mr Blows points out.
The student unions agree. "Overseas students generate a vast amount of money for the UK," says Mr Horscroft. "It is a real own goal. The international markets for graduate students are very active. Telling research students that the UK education market does not want them is a really bad move."