Major farms and factories in Britain which supply supermarket chains such as Asda, Sainsbury's, and the up-market Harrods Food Hall, are using illegal immigrants, according to an undercover investigation.

A Panorama programme to be broadcast on BBC1 tonight, shows gangmasters exploiting workers from Eastern Europe and leaving them to live in poverty.

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

One illegal immigrant tells the programme: ''I would have got out of here the next day. But I did not have any money - I even had nothing to eat.

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

The programme says the supermarket price war is driving the trade in illegal immigrants.

Trade Union researcher Don Pollard tells it: ''The supermarkets buy 80% of the fresh produce in this country and they're putting tremendous pressure on both the packhouses and farmers to reduce costs.''

All the suppliers and super-markets featured in the programme say they have no knowledge of immigrants being employed and say that legal responsibility does not lie with them.

The programme infiltrated the secret labour forces being sent to work in four large suppliers across the country, processing goods from chickens and pies to lettuce and potatoes.

Many illegal immigrants come to Britain to work in the hope that they could return home with money for their families, answering advertisements in Eastern European newspapers offering highly paid work and good conditions.

Often the immigrants would pay more than #1000 for the promise of transport to Britain and work when they arrive.

In the film they are sent to work in factories and farms supplying supermarket chains which include Asda, Sainsbury's, Iceland, Harrods, Morrisons, Aldi, and the Co-ops.

On arrival in this country workers are sold forged asylum-seeker documents and charged to be introduced to gangmasters who find them work. Once in the gangmasters' grasp they are underpaid and exploited, and can be left in debt when work dries up.

Christine Watts, corporate affairs director at Asda, said: ''We were horrified when we found out that one of our packhouse suppliers had been using illegal immigrants as casual labour.

''We took immediate and very firm action to commission an independent ethical audit of the supplier concerned. This did throw up some issues which we are now working with the supplier to put right.

''We have a very robust and demanding ethical trading policy which involves an adherence to immigration and employment laws.

''What we don't do is say we are not going to work with the sup-plier, as that wouldn't resolve the problem - only push it elsewhere. We are working with the supplier to improve the situation.

''I know Panorama is talking about a link between supermarkets demanding cheap food and suppliers opting to use illegal immigrants, but we have not yet seen any evidence connecting pricing policy to employment.

''The supplier in question has told us that the agency costs for illegal immigrants was no less than other agency staff. So it appears it is the unscrupulous gangmasters who are making money.''

A spokeswoman for Iceland stores said: ''Panorama brought this to our attention and we were surprised as it was one of our suppliers who we visit regularly.

''This is taken incredibly seriously and we have launched an investigation of our own.

''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.''

A spokesman for Sainsbury said: ''Obviously we take this sort of thing very seriously.

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

''We would be very concerned if these practices were going on and will be seeking more information and speaking to suppliers.''