The UK visa system for students may have been exploited by as many as 50,000 migrants, during the first year of the new rules.
A recent report from Whitehall's spending watchdog the National Audit Office found that the 'key controls' were not put in place in the system following its introduction in 2009.
The changes should have meant that each student was required to be sponsored by a licensed college and that no individual would be able to change the institution they attended without permission.
But the system was introduced before the required controls for implementing these changes were in place. As a result, "in its first year of operation, between 40,000 and 50,000 individuals may have entered the UK … to work rather than study".
The report added: "The agency has taken little action to prevent and detect students overstaying or working in breach of their visa conditions because the agency regards them as low-priority compared to illegal immigrants and failed asylum seekers."
The current coalition government has since introduced a number of "radical reforms" to the UK immigration system in a bid to stamp abuse such as this and "restore order to the uncontrolled student visa system we inherited", immigration minister Damian Green observed.
He explained: "These include tough new rules on English language, working rights and dependants to ensure only legitimate students come to the UK. New restrictions on post-study work mean that all but the very best will return home after study."