From April, the £3m spent on embarkation controls - described as "routine and unproductive" by Home Office Minister Mike O'Brien - will be used to try to detect the rising numbers of people entering the country illegally.

Passports will be checked by airlines

But the Immigration Service Union has criticised the move, saying it will be harder to ensure that illegal immigrants ordered out of the country have actually left.

The union blames the general lack of effectiveness of embarkation controls on under-investment by the government.

Mr O'Brien said the current system of checking passports as people leave Britain was an "expensive fiction", as no records were kept of who had left.

In 1994 embarkation controls were abandoned for those visiting European Union countries from ferry ports and small airports, which means 40% of travellers leaving the country do not see an immigration officer.

"Present controls on leaving the country are ineffective, and that is why we must better target resources," said Mr O'Brien.

"We have therefore decided to replace the residual embarkation controls with an intelligence and target-led operation, involving a partnership between enforcement agencies, carriers and port authorities."

Closed-circuit TV cameras will be used to film people leaving and facilities will be retained to allow full checks to be mounted during special alerts.

Mr O'Brien said the cuts would have very little effect on stopping estranged parents taking their children abroad illegally.