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The number of foreign students let into the UK is "unsustainable", immigration minister Damian Green has said.

In a speech, he questioned whether institutions were attracting the best students - with only half of student visas issued for university courses.

Home Office research suggests a fifth of students are still in the UK five years after being granted visas.

But the National Union of Students dismissed Mr Green's comments as a "misinterpretation" of the facts.

The Home Office study tracked non-EU migrants who came to the UK in 2004. The largest group - some 185,000 people - were students, and 21% were still in the country five years later.

´óÏó´«Ã½ home affairs correspondent Danny Shaw says this, together with an increasing number of new overseas students, has led Mr Green to make reform of the student immigration route a priority.

Ministers also intend to examine work visas as two-fifths of people in this group remained in the UK after five years.

Ahead of his speech, Mr Green said: "We can't assume that everyone coming here has skills the UK workforce cannot offer."

He told ´óÏó´«Ã½ Radio 4's Today programme: "I don't want to interfere with the success stories of our universities."

But he said there was a need to examine closely sub-degree courses and the reasons for students remaining in the UK.

Mr Green said: "Why are they staying on? What are they staying on to do? This is part of a wider look we need to take at the immigration system."

Office for National Statistics figures released last month showed net migration to the UK increased by 33,000 to 196,000 in 2009.

The number of visas issued to students went up by 35% to 362,015.

Mr Green said the figures were proof the coalition government had inherited an immigration system "largely out of control".

In his speech at the Royal Commonwealth Club, Mr Green said an annual cap on economic migrants from outside the EU would not be enough to reduce net immigration to the "tens of thousands".

Internal Home Office estimates showed more than 90,000 people were coming into Britain every year to do courses below degree level at private institutions, Mr Green said.

"The foreign students attending these various establishments may, or frankly may not be, the brightest and the best.

"I want to ensure those who come here to study at language schools or any other institutions play by the rules and leave when their visas expire.

"We need to decide whether this is right and also whether it is the best thing for the students themselves, given the high financial commitments required of them."

But Aaron Porter, president of the National Union of Students, said: "To suggest that the levels of those coming to the UK to study is too high is a politically motivated misinterpretation of the huge contribution which international students make to our colleges, universities and the economy.

"The government should be proud that students choose to study in this country creating an education industry worth £12.5bn a year to the British economy."

Sally Hunt, general secretary of the University and College Union, said: "Populist policies on immigration might play well domestically, but on the global stage we risk looking foolish.

"Damian Green is making his speech today after returning from a trip to India where he encouraged students to come to the UK."

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