The Home Office has suspended English language tests run by a major company after a TV investigation claimed that Britain's student visa system is riddled with fraud.
BBC One's Panorama programme said it had found blatant and routine cheating in Government-approved exams and a thriving market in false documents enabling people to stay in Britain illegally.
Researchers found that, for a fee, criminal immigration agents can help people get around language tests which are necessary for a visa to be obtained - even if they speak little or no English.
One immigration consultancy business in London offered a guaranteed pass for a £500 fee, before sending one of the programme's undercover students to sit the English exam at a college designated as "trusted" by the Home Office.
Undercover footage shows the student making her way to a secure computer terminal linked up to a Government-approved testing company to take a test accepted by the Government for visa applications.
Minutes before the exam was due to start, new people entered the room. In the footage, the dozen or so candidates in the room are seen standing aside from their desks to allow these 'fake sitters' to take the exam for them.
In the footage the 'fake sitter' taking the BBC researcher's exam is seen answering questions in the written and oral papers in perfect English. Meanwhile the registered candidates are seen standing quietly in the aisle waiting their turn to be called to the front of the room to have their photo taken by an invigilator to 'prove' they have sat the test.
The undercover researcher returned to the same test centre a few days later and filmed the second half of the exam, the multiple choice.
In the footage, the invigilator is seen simply reading out the answers to all 200 questions so the registered candidates can copy them down. A two-hour test took just seven minutes to complete.
A few days later, the researcher received a genuine exam certificate showing she had passed with high marks.
Panorama said tests are run by ETS, one of the biggest English language testing companies in the world.
The programme said it had also filmed in a second secure exam centre where a similar fraud took place, and that another immigration agency had also offered a forged document to help secure a visa extension of at least a year.
Home Secretary Theresa May said: "For too long many colleges, particularly private or further education colleges, have been selling visas and not education. It is time for them to face up to their responsibilities as purveyors of education and not abuse.
"The student visa regime we inherited was open to widespread abuse. It neither controlled immigration nor protected legitimate students from substandard sponsors. Our reforms have curbed abuse by closing bogus colleges, making the application process more rigorous and imposing more rules on colleges to improve course quality.
"However, as Panorama has highlighted, much more needs to be done. This type of abuse is not acceptable and as criminals, bogus colleges and economic migrants seek new ways to exploit the system we will continue to change our methods to clamp down on them.
"We have taken action and suspended the two colleges identified in the programme. Applications made by students in the UK using the English Testing Service or associated with the colleges or immigration advisers mentioned in the programme have been put on hold pending the outcome of those investigations.
"All further English language tests done through ETS in the UK have been suspended."
Shadow home secretary Yvette Cooper said: "This investigation shows Theresa May is presiding over a failing immigration system which too often focuses on the wrong thing and where illegal immigration is a growing problem.
"We have warned Theresa May repeatedly that short term student visitor visas which have weak checks are being abused. We have also told her that thousands of warnings about false students are not being followed up by UK Border Agency.
"The Home Secretary extended short term student visitor visas and ignored their abuse because they aren't included in her net migration target - and yet she is discouraging genuine graduate students, who are included, even though they bring in billions of pounds of investment.
"Theresa May promised she would tackle bogus students but allow the brightest and best to study in the UK. Yet
once again, the rhetoric doesn't match the reality. Instead abuse is getting worse, whilst genuine international graduate students are being put off."
ETS was not immediately available for comment but the company told Panorama: "ETS does everything it can to
detect and prevent rare instances of dishonest test administrators or test takers."
A notice on its website in the UK said that the Home Office has requested ETS to suspend various tests temporarily in the UK related to immigration purposes.
Candidates who had appointments to take a test for immigration purposes would be contacted to process a refund.
Immigration Undercover: The Student Visa Scandal will be broadcast at 8.30pm tonight on BBC One.