Britain is to tighten the rules on immigrants entering Britain on a student visa, the government said on Sunday in a clampdown on a system which some security experts say has been exploited by Islamist militants.

Home Secretary (interior minister) Alan Johnson said the crackdown was part of a wider campaign against immigrants who apply for student visas even though they intend to work.

The tighter controls could also help to tackle security concerns over militants who enter Britain ostensibly to study. Analysts have warned for years of a threat from Islamist militants based at British universities, including foreigners on student visas.

The issue climbed back up the political agenda last month when it emerged that the Nigerian man accused of plotting to blow up a passenger plane over Detroit tried to re-enter Britain last April to study at a bogus college.

In a counter-terrorism operation last April, police arrested 12 people including 11 Pakistani nationals, all but one of whom were on student visas. Prime Minister Gordon Brown described the case as a "very big terrorist plot", but police released all the men without charge. Eleven were later investigated by immigration officials.

Johnson said that nearly a third of immigrants seek to enter Britain on a student visa and that the country is the second most popular study destination in the world. The government has closed down 200 bogus colleges, which help students into Britain but don't offer proper courses.

A Home Office spokesman would not confirm how many student visas are expected to be cut each year. Britain issued 236,000 student visas in 2008-09 and refused 110,000 applications.

Under the new rules, applicants from outside the European Union will need to speak better English and will face tougher restrictions on taking part-time jobs. Immigration has long been a source of criticism for the ruling Labour, behind in polls before an election due by June.

Opposition Conservative leader David Cameron has accused ministers of allowing an unsustainable number of immigrants into Britain and has proposed a cap to keep levels down. Conservative home affairs spokesman Chris Grayling said the government was "floundering around trying desperately to correct their own mistakes".