In a statement, Johnson said Britain will continue to welcome genuine foreign students, but will come down heavily on those who use student visas to come to Britain mainly to work and abuse the system.

He said: "We remain open to those foreign students who want to come to the UK for legitimate study - they remain welcome. But those who are not seriously interested in coming here to study but come primarily to work - they should be in no doubt that we will come down hard on those that flout the rules."

Under the new measures, successful applicants from India and other countries outside the European Union will have to meet a higher bar on English language requirement, and students taking courses below degree level will be allowed to work for only 10 hours a week, instead of 20 as at present.

Those on courses which last under six months will not be allowed to bring dependants into the country, while the dependants of students on courses below degree level will not be allowed to work.

Student visas for courses below degree level will be granted only if the institutions they attend are on a new register, the Highly Trusted Sponsors List.

Officials said they suspended issuing student visas in north India, Nepal and Bangladesh after the system had been overwhelmed amidst concerns that many cases were not genuine.

The new rules a review ordered by Prime Minister Gordon Brown after he said the alleged Christmas Day bomber, Umar Farouk Abdulmutallab, who had studied in London, had linked up with al-Qaeda in Yemen after leaving the UK.

In 2008-9, about 240,000 student visas were issued by the UK.