They are the slaves of the food sector – illegal immigrants often being paid a pittance at suppliers to some of our biggest retailers. As the coalition turns up the heat, Elinor Zuke asks: is enough being done to stamp out the abuse?

They have knives. And we're told they might use them on us.

So when the 30-strong UK Borders Agency taskforce storms into the warehouse only to be greeted by a handful of bewildered workers, it's a bit of an anticlimax. But the intelligence proves correct. Today's three co-ordinated raids on the West London warehouses of an Indian food supplier will unearth eight staff members working in Britain illegally.

The supplier is Chhappan Bhog. And it has form. Last year the company was fined £20,000 when four illegal workers were found preparing food destined for Tesco and a number of airlines.

Despite the fine and Tesco's insistence it "expects all its suppliers to abide by the law", the retailer continued using the company, and the company continued hiring illegal workers. If it is to avoid another, far heftier fine, Chhappan Bhog must now prove it wasn't negligent in employing the eight workers.

As the officers question the workers back in West London, the extent of the problem begins to emerge. One young woman fights back the tears as she is questioned on the working conditions and the hours she's been putting in at Chhappan Bhog.

She doesn't understand the questions, she says, she's merely a part-time worker studying in Britain for an MBA. Officers are told by shaken staff that they've been working for up to 10 hours a day for as little as £30 a shift almost half the minimum wage. The company denies the claims, which are still being investigated.

The shame of the food industry is that allegations of this nature are not uncommon. Nor are they restricted to the casual gang labour covered by the Gangmasters' Licensing Authority (GLA). According to provisional figures from UKBA, the food sector accounted for 88% of the 788 civil penalties doled out for employment infringements in the three months to 30 June.

Restaurants and takeaways accounted for the lion's share of the £8.4m in fines handed out to the industry in the first six months of 2010, and retailers, wholesalers and grocery suppliers were fined almost 10% of the total (£830,900).

The shocking record in this sector has made food the number-one target for the authority in coming actions. 

"These figures show that among a significant number of businesses in the food sector, recruitment practices are not what they should be," says Rebecca Baumgartner, UKBA enforcement officer for West London. "Far too many are not carrying out the checks on staff that they need to by law."

A number of suppliers to the biggest supermarkets are now under investigation. But is the approach actually working, or as critics claim is it merely "scratching the surface"?

Immigration has been one of the most contentious policy areas for the new government, but the parties have so far avoided an impasse. Their joint manifesto signalled an end to Britain's "porous" border policy with the introduction of a cap on numbers entering from non-EU countries, the formation of a Border Police Force and an assurance that no amnesty will be given to illegal immigrants already in Britain.

To hammer this new harder line home, UKBA has been ramping up raids on those suspected of employing illegal workers, with a particular focus on the food sector. Preliminary figures for July suggest 584 raids were carried out on suspected illegal working operations, up 31% year-on-year.

"The main problem is that Britain has got a reputation around the world as a place where it's really easy to work illegally and that's the reputation I'm very keen to change," says immigration minister Damian Green. This summer's raids are part of that push to change perceptions. "It's about using the powers we have effectively and in a targeted way," adds Green.

But the efforts are not enough, according to critics. "It's a revolving door," says one union source. "There needs to be a radical rethink. Employers should be fined and repeat offenders should be jailed I don't see a problem with that. It's the individuals who are the victims."

"They're barely scratching the surface," agrees Richard Dunstan, social policy officer at the Citizens Advice Bureau. "The UKBA's focus is just to arrest as many illegal workers as possible, because that's the target set by ministers. The priority should be to tackle employers. At the moment the UKBA is just running to keep still."

The issue is one of legal power. Presently the UKBA can impose a fine of up to £10,000 for every person found working illegally for a company. But, according to Dunstan, such fines do not work as a deterrent. "At the moment companies that have been given a civil penalty can carry on trading in exactly the same way. Stop Now orders can stop a company from working immediately and a breach is criminal so it means the company has to stop." But presently the UKBA has no power to issue Stop Now Orders.

Jim Sheridan - the Labour MP who introduced the Gangmasters Licensing Bill in the wake of the 2004 Morecambe Bay cockle-pickers tragedy - claims the time has come for the GLA's remit to be extended to cover employees hired directly by food manufacturers.

"We've solved many of the issues in the agricultural industry, but the problem has moved to other industries," he says. The real power to stamp out the exploitation lies with the supermarkets, says Sheridan, and they should be wielding that power.

"Suppliers should be made aware they could be visited at any time and if they're exploiting workers the contract will be taken away. That's the only thing that will concentrate their minds. We need a tangible audit trail to say 'this food came from x and got to z and people weren't exploited in between'."

Of course all of the big five multiples say they expect their suppliers to stick to the letter of the law and claim they will act to address issues as they arise. But it remains unclear how retailers actually ensure their expectations are met. Only Asda says it carries out regular audits.

Home Office figures also suggest symbol groups could be doing more to stamp out the use of illegal labour. In the first six months of the year retailers operating under the fascias of Costcutter, Londis, Nisa, Best One and Spar were fined a total of £60,000 after illegal immigrants were found working behind their counters.

"When someone buys into a fascia group you buy their products, their promotions and the marketing, but the way it's run is the entire responsibility of the shop, or the member," says a spokesman for Best One, which had one of its members fined £5,000 for employing one illegal worker. "It's entirely up to them and you'll find this is the way for all symbol groups."

Not quite. A spokesman for Costcutter, which had two retailers fined £5,000 each for employing illegal workers, says: "In the event of any retailer being found to have employed illegal workers, Costcutter would investigate the matter and take appropriate action to protect our brand."

Back in West London, the people found working illegally at Chhappan Bhog are being led from the warehouse. The young woman is among them. She's been working over the hours she's permitted to as a student, thus invalidating her visa. Tears stream down her face and she is visibly shaking. "She was hyperventilating before," says an officer as he leads her to the van that will take her and her colleagues to a detention centre. From there they will be deported to their native India. So far, four are already there.

Chhappan Bhog, which turned in a loss of £228,000 last year, will face yet another fine if it cannot prove it took all necessary steps to ensure its workers' eligibility to work in the UK. It continues supplying Tesco, and plans to fight the UKBA's actions in the meantime. Tesco is refusing to comment on the action, if any, it will take against its supplier.

Would it be so silent if the raids had been conducted in a sweatshop in the Far East?

On the trail of illegal labour
10am: A 30-strong UKBA taskforce storms the West London premises of Chhappan Bhog, acting on intelligence that illegal workers are employed here. We swoop as teams raid two other Chhappan Bhog sites across the capital.

10.15am: Several people, all originally from India, are discovered working in the kitchen inside. The officers identify themselves and begin questioning the workers. No one is allowed to leave.

10.30am: The interrogation continues in the premises' offices. The officers radio through each of the workers' details to the UKBA headquarters, which are checked against immigration records.

11.15am: Documents are inspected and stories are checked out. A number of workers claim they have been working for Chhappan Bhog for as littles as £3 an hour. A number of workers are found to be ineligible to work in the UK.

11.30am: The first arrest of the day that of a young woman who claims to be in the UK studying is made. The woman is led from the premises to a waiting van. She could face weeks in a detention centre before deportation.

12.00pm: Four illegal workers are arrested, bringing the day's total to eight. Chhappan Bhog faces a fine of up to £80,000 if it can't prove it took appropriate steps to ascertain the workers' rights to work.