The 15 men – from Iraq, Afghanistan and India – were found hiding in the back of a lorry bound for Northampton when it pulled into the port of Calais.

After stopping the lorry, which had come from Poland and was carrying light bulbs, officers from the UK Borders Agency used a hi-tech carbon dioxide probe, which detects breathing, and found the stowaways.

A spokesman for the Government agency said: "It's important we stop would-be illegal immigrants before they reach the UK.

"That's why we've got hundreds of British immigration officers based at ports in France and Belgium."

The spokesman added: "Last year, we searched over a million lorries and prevented 18,000 foreign nationals from crossing the English Channel without permission."

As well as visual checks of vehicles and carbon dioxide probes, immigration officers also use heartbeat detectors and sniffer dogs to find well- hidden stowaways.

The problem of illegal immigrants trying to make their way to Northampton was first highlighted in September last year.

At the time, the Chronicle & Echo reported on two cases where people from both Iraq and Africa were found hiding in the backs of lorries driven into the county.

Also last year, statistics released in the House of Commons showed Northamptonshire Police had dealt with illegal immigrants being smuggled into Northamptonshire in the back of lorries an average of once a fortnight.

To help tackle the stowaway problem, mobile prison vans will be used in the county this autumn.

The vans will be similar to ones already used in Dorset, which are designed to hold up to 10 suspected illegal immigrants for up to 12 hours while their details are checked.

It is believed Northamptonshire is a hotspot destination for illegal immigrants because of the county's location at the centre of the country's transport network.