The men broke out of the Oakington Immigration Centre in Longstanton, Cambridgeshire.

Sources said a group of four young Vietnamese men cut through razor wire fences after crawling through a metal service duct similar to those used for air conditioning.

The men, who escaped from the overnight lock-up between Thursday and Friday, were being detained for offences of cannabis production and possessing forged documents.

Later it emerged three further detainees, whose identities are unknown, had fled the centre on Sunday.

A spokesman for the UK Border Agency said: "We are determined to capture these seven detainees. Razor wire and patrol dogs are now in place at Oakington."

The centre, a former RAF base, can hold up to 352 men and opened in 2000.