Organised crime networks have set up phoney companies and employment agencies to abuse the "points-based" visa system, introduced under Labour in 2008, which puts responsibility on employers to "sponsor" migrant workers.
The Serious Organised Crime Agency (Soca) warned that the tactic allowed foreign criminals to be brought into Britain legitimately on work permits.
Scotland Yard has said that around half of London's organised crime gangs are "cultural networks" whose members share a foreign nationality - typically from countries such as Turkey, China and Vietnam whose citizens require visas to come to Britain.
It came as photographs were released demonstrating the lengths to which some illegal immigrants will go to enter Britain.
Soca said that fraudulent work permits and student visas, along with bogus marriages and exploitation of visas for family visitors, were "common methods of abuse" following the introduction of the points-based immigration system by the Home Office in November 2008.
"There is a new emphasis on placing responsibility on sponsorship to determine applications," said a briefing document from Soca.
"This ... in turn presents potential opportunities for organised criminals to provide false documentation which enhances or creates sponsorship profiles."
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