But all the supermarkets involved deny knowing that illegal immigrants were being used and have pledged to investigate the matter.

I would have got out of here the next day, but I did not have any money - I even had nothing to eat."

Illegal immigrant worker

Panorama journalists infiltrated the secret labour forces being sent to work at four large suppliers across the country.

They processed goods ranging from chickens and pies to lettuce and potatoes.

Many of the immigrants are tempted to work in Britain by advertisements in Eastern European newspapers that offer highly paid work and good conditions. Often they will pay more than £1,000 for the promise of transport and work.

But on arrival in this country, workers are sold forged asylum-seeker documents and charged to be introduced to gangs who find them work, exploit them and leave them in debt when the work dries up.

Two Estonians describe how they each pay £60 a week for a flat which has a bare concrete floor and was so cold at night that their clothes froze.

Asda and other supermarket chains deny knowing their suppliers used immigrants

"I would have got out of here the next day, but I did not have any money - I even had nothing to eat," said one of the men.

"So I was without food, beans without bread, looking for cigarette butts on the street. With this sort of life one can hang himself."

All the suppliers and supermarkets featured in the programme say they have no knowledge of immigrants being employed and say they are not legally responsible.

Christine Watts, corporate affairs director at Asda, said the supermarket chain was horrified when Panorama revealed that one of its suppliers had been using illegal immigrants as casual labour.

"Robust trading policy"

"We have a very robust and demanding ethical trading policy which involves an adherence to immigration and employment laws," she said.

The firm is now working with the supplier to improve the situation rather than end their contract and force the problem elsewhere.

"The supplier in question has told us that the agency costs for illegal immigrants was no less than other agency staff," said Mrs Watts.

"So it appears it is the unscrupulous gangmasters who are making money."

Iceland stores were also surprised that one of their regular suppliers featured in the investigation

It appears it is the unscrupulous gangmasters who are making money

Christine Watts, Asda

"We are waiting to see the programme, and if the supplier acted illegally or otherwise at odds with our own ethical position they may be de-listed," said a spokesman.

A spokesman for Sainsbury said the firm took the matter very seriously.

"We have a number of supplier support schemes and work closely with the National Farmers Union as we believe in fair trade," he said.