Illegal immigrants will be rounded-up by roaming prison vans

The "mobile detention centres" will aim to catch recently arrived foreigners as they emerge from peoplesmuggling lorries.

Immigration officers will hold the suspects inside the vans until background checks are performed.

If they are found to be here illegally, they will be taken by police to a major detention centre in Oakington, Cambridgeshire, before being repatriated.

The sheer number of bogus arrivals has meant police have been too busy to do the job.

The first vans are due to be launched in Northamptonshire following a successful try-out in ports along the South Coast.

The vehicles were ordered by Immigration Minister Liam Byrne after the Government was embarrassed by two incidents last September.

Police caught 16 illegal Iraqi immigrants leaving a lorry in Flore, Northamptonshire, but, instead of alerting the authorities, told them to travel almost 100 miles to a detention centre in Croydon and sign on as asylum-seekers.

The previous week, five African men had been found in a lorry in Long Buckby, Northamptonshire.

On that occasion, the police actually gave them a lift to a railway station before asking them to catch a train to Croydon.

Northampton North MP Sally Keeble, who had raised the incidents with Mr Byrne, said: "They were stupid situations - no one would expect desperate people to travel halfway across the country to hand themselves in.

"These vans are a good idea and will help to take the pressure off local police and services."

New Government figures reveal that the number of illegal stowaways has more than doubled in just three years. In 2003, the number of people found entering the country clandestinely was 3,127. By 2006, it was 7,552.

Shadow Immigration Minister Damian Green said: "We will need to see whether this measure changes much in practice or whether it is another gimmick."

But last night, questions were being asked as to how staff at the Border & Immigration Agency would find the time to roam the streets in prison vans.

A leaked secret Government memo last week revealed that immigration officers had been ordered to stop deporting foreign students who overstay their visas, suggesting that they were too busy to do so.

And last month, a leaked document from the Prison Service revealed that immigration bosses had "no interest" in deporting foreign prisoners who had served less than a year behind bars.