Lord Green is Chairman of MigrationWatch UK.

As the political season gets underway, immigration is yet again one of the very top issues of public concern and the Home Secretary has come out fighting.

For the first time she has been specific about the likely numbers in which students from outside the EU are staying on in Britain. Her figure of 96,000 a year has been immediately challenged by David Willets, the Minister of State responsible for Higher Education in the coalition government.

This is not just a matter of some obscure statistics. It is central to the government’s efforts to get net migration down to acceptable levels.

It is not in dispute that the official immigration figures have shown for several years that non-EU students are arriving at an average of around 150,000 a year but only about 50,000 a year are recorded as leaving.

The incoming figure must be about right as it is broadly consistent with the number of visas issued (although somewhat lower because some students change their minds or have come for short courses and do not qualify as migrants).

The outgoing figure has, in recent years, been based on a question about the purpose for which the migrant arrived so it is clear that it is departing students who are being counted.

The graph below shows the large increase in arrivals under Labour’s plans for the expansion of the sector but no subsequent blip in departures:

David Willets claimed on Radio 4 that “better and more reliable” research by the Home Office between 2007 and 2012 had indicated that just two per cent of students failed to comply with their visa requirements. In fact, this was a Border Agency analysis, carried out in 2010, and the two per cent applied only to university students – not to the colleges where most of the abuse is believed to have occurred.

Indeed, it is significant in this context that visa applications to study at further education colleges have fallen by 85 per cent since 2011. By contrast, foreign students at Russell Group universities are up by 33 per cent since 2010.

We all agree that foreign students are an important source of revenue for higher education, and that their presence is also valuable in a social sense, adding to the mix of the student body.

That explains why there are no limits on the number of genuine students who can come to the UK to study: indeed the number of applications for study at all UK universities has increased by 17 per cent since 2010.

Some who stay on with work permits are also valuable to our economy, although the take up by employers has been remarkably low – only 5,600 in 2014. In the wider context, however, students comprise more than half the non-EU inflow, so they must be properly accounted for.

The key point is that students are counted as they arrive in the UK and are also counted out as they leave. So, if they are indeed returning after their courses, they will have no effect on net migration. There is the rub. At present the evidence suggests otherwise and, if it is correct, non-EU students are a very significant part of net migration.

At present the numbers are based on a passenger survey so they will not be precise. However, exit checks are now coming into effect and will produce results in six to nine months that will show more accurately what is happening.

The Higher Education lobby may well be awaiting them with some anxiety, since many of the arguments for foreign students depend on their returning to their home countries.

Yet, obviously, if large numbers are not returning they are not contributing to the UK’s “soft power”, as often claimed. They are, instead, adding to our population, and as a result, to the pressures on our public services.

Indeed the student route risks becoming a back door to Britain, if it is not already.

There is also the very important matter of reputation. In 2008 about 50,000 bogus students applied from the Indian sub-continent. In subsequent years some 870 “bogus” colleges have lost their right to sponsor foreign students. It is clear that the student route has been seriously abused.

It is surely in the longer term interest of the sector itself for this to be dealt with rather than for the issue to be ducked.

Some 60 per cent of non EU migrants arrive by the student route. Calls to take them out of the net migration target are entirely misplaced. This would deeply undermine the credibility of the government’s efforts to reduce net migration.

Last month public concern about immigration reached its highest level ever. This is absolutely not the time to favour sectoral interests at the expense of further damage to the government’s credibility with the British public.