By Brian Wheeler BBC News Online Magazine

Low cost groceries are just one of the benefits of cheap labour

The UK's army of illegal migrant workers may be largely "invisible", but the fruits of their cheap labour have become an indispensable part of our modern consumer lifestyle.

Home Secretary David Blunkett has promised a crackdown on rogue gangmasters who prey on desperate immigrant workers.

But what is perhaps more shocking is the extent to which some sectors of British industry - from manufacturing to food production and construction - have come to depend on cheap foreign labour.

Many of the things we take for granted - cheap fresh food in supermarkets, affordable designer clothes - rely, to a greater or lesser extent, on the exploitation of foreign migrants, working legally or illegally in the UK, often in appalling conditions.

Official figures are hard to come by, but some estimates suggest there are as many as half a million illegal migrant workers in the UK, mostly concentrated in the South East. Many more are here legally, but earn less than the minimum wage and are forced to work in dangerous conditions, with no training.

They are often exploited by the shadowy middle men - so-called gangmasters - and employment agencies that have sprung up to service the flexible labour demands of the multinationals.

INVISIBLE WORKFORCE The 2002 film Dirty Pretty Things (above) lifted the lid on illegal working in London's hotel trade In 1999, Britain expelled some 6,000 illegal workers They are commonly found in flower picking, fruit and veg harvesting, frozen food processing Panorama investigates gangmasters (June 2000)

Sweatshop labour and semi-slave hours - that many assumed died out in Victorian times - are a fact of life for such workers.

It has been estimated, for example, that one in four workers in London's hotel and catering trade is an illegal migrant. On Christmas Day, illegal Ukrainian migrant Roman Kobitovich was found dead in a cupboard in The Cafe Royal, one of London's smartest restaurants, after apparently suffering a fall.

It emerged Mr Kobitovich, a highly trained engineer, had been working as a kitchen porter and living a virtually feral existence in the bowels of the restaurant so he could send money back home to pay for his daughters' education.

But many of the worst abuses take place in rural areas, hidden from the authorities.

David Blunkett said on Monday he would back a private members bill by Labour MP Jim Sheridan that would help stamp out some of the worst cases of exploitation. But ending agriculture's addiction to illegal labour may prove more tricky.

"It is very difficult to avoid using illegal labour," says one Lincolnshire farmer, who supplies major supermarkets.

Tarnish brand

Over the past two years the supermarkets - which may at one time have turned a blind eye to potential exploitation - have toughened up screening procedures.

"They are extremely anxious to not have their brand tarnished," says the Lincolnshire farmer. "We are also extremely anxious not to have our brand tarnished."

The construction industry is rife with illegal labour

But workers, more often than not from former Soviet states, are determined to slip through the net and there is still a clear demand for them.

As High Street names become more scrupulous in their vetting procedures, illegal labour is being pushed to the margins, making it harder to identify. The fact is, another farmer pointed out, a labourer from the Ukraine will work a lot harder, for less money, than local British workers.

Cutting out illegal labour altogether would probably force up shop prices, says another farmer, or certain crops "would go unharvested".

The textile trade has also seen an explosion in illegal, underpaid labour according to Baz Morris, of textile workers union KFAT. The informal sector, which includes home workers and backstreet sweatshops, is "probably as big" as the above-board sector.

The construction industry is also struggling to get a grip on the problem.

Shock to economy

If all the illegal construction workers in London were rounded up and sent home tomorrow, it would send shockwaves through the capital's economy, says George Brumwell, of the industry union UCATT.

Each time there's a spot check they'll always pick up a few illegals - you'll never find a building site full of them, that would be too conspicuous George Brumwell

"Of the 200,000 building workers in London, probably 40,000 are migrants and a fair proportion are illegals. It's a dirty, physically tiring job - perhaps you might say the last big manual heavy industry in Britain," he says.

Wages can be horrendously low. Four years ago, 11 Indian stonemasons were found working on a temple in west London, some for as little as 30p an hour.

Yet the problem is getting worse, says Mr Brumwell.

"Each time there's a spot check on a big site they'll always pick up a few illegals. You'll never find a building site full of them, that would be too conspicuous.

Mr Brumwell's union consistently fights illegal labour.

But he admits that while the construction industry remains a driver of the economy - worth �70bn a year - coupled with the skills shortage in the UK labour market, contractors will be tempted by illegal foreign workers.