Pictured: The fleet of 'mobile cages' to round up illegal migrants ... it's just one truck

A fleet of detention lorries would roam the streets, equipped with "cages" to hold those they found.

No longer would there be the farce of suspects being handed a map, and told to make their way to a detention centre because there was nowhere nearby to hold them, ministers said.

Lone crusader: The detention lorry, and a van used to ferry migrants from the truck to custody

So proud were they of the mobile detention centre policy that it was unveiled with great fanfare at the Labour Party Conference in Bournemouth last September.

However, as with so many good ideas, it has yet to really get off the ground. Or, more specifically, it has yet to leave the Bournemouth area.

Eight months on, there is still only one lorry  -  and it has not strayed far beyond neighbouring Poole.

Last night Tory immigration spokesman Damian Green said: "Yet again the Government is caught out talking tough but acting weak.

"Ministers wanted us to believe that a fleet of these vehicles would make a real difference to the fight against illegal immigration.

"Now we know there is only one, based in an area which is not the busiest point of entry.

"This shows why we need an integrated Border Police Force, so that the protection of our borders is run more efficiently than the current chaotic system.

"After 11 years, this Government has still failed to get to grips with border protection."

The lorries, officially named mobile detention centres, were a response to the controversy over illegal immigrants being caught by police, only to be released again.

Officers were contacting immigration centres, but being told no one was immediately available to pick up the suspect.

As a result, police were told to let them go with a map to the nearest immigration detention centre.

Predictably, many vanished on the way. Last September, unveiling the first of the lorries, Immigration Minister Liam Byrne said the vehicles could be driven to where they were needed most, holding up to ten suspects for 12 hours at a time.

Mr Byrne said: "Tougher checks at Britain's borders mean on the spot lock-ups are now essential.

"It sends a very clear message to would-be illegal immigrants that when we catch you we'll instantly detain you and then we will send you home."

In March, he said a fleet of mobile detention centres would be sent out, adding: "They're big trucks with cages in."

But a Parliamentary question, recently asked by Mr Green, paints a different picture.

It revealed there was only one truck, which had been involved in a pilot scheme limited to small South Coast ports, "primarily Poole".

A different "specification" of the truck is needed, officials have decided, and this is "currently being developed".

The lorry is understood to have cost as much as £100,000, and was manned by four custody officers.

It was taken out of use in February, so the results of its work could be evaluated by ministers. A van was used with the lorry to ferry immigrants to detention centres.

Mr Byrne said a new vehicle is to be piloted in Northampton in the autumn.

He declined to say how many illegals had been rounded up using the lorry, how many hours it had been in operation for, or how much cash had been spent.

Last night, a UK Border Agency spokesman said: "Having committed to doubling our enforcement resources we have been testing new and innovative ideas.

"The pilot in Poole has demonstrated a number of benefits and as a result we will be rolling out an updated model in Northampton."