At first glance it could be any building on any industrial estate in Britain. The cavernous, prefabricated warehouse on the outskirts of Preston, a stone’s throw from a Halfords and opposite a timber merchants, is a study in anonymity, a place you would visit only if you needed something in particular.

Outside its foyer, in large, concrete tubs that house palm trees struggling in the Lancashire autumnal chill lie several ornamental metal chickens. A small sign on the window of the security guard’s office at the edge of the compound proudly asks, with accompanying glossy photographs: “Why not try our chicken burgers? Chicken tikka pizza, chicken sausages and chicken nuggets. Authentic halal you can trust. Available in store.”

The workers pouring in and out of the plant on any night of the week confirm that this is a big operation, a place that processes a million birds a week.

The Mercedes and Range Rovers parked outside the warehouse, all with similar, personalised registrations, suggests that Gafoor Poultry is a profitable family business. Even the company’s lawyer drives a top-of-the-range Audi.

The lawyer was busy last Friday afternoon when, just after 4pm, he received a call to tell him that almost 60 Home Office enforcement immigration officers, accompanied by several members of the Lancashire police force, had raided Gafoor’s premises in an operation months in the planning. Immigration staff from Manchester and Liverpool airports had been bussed in for the raid and were taken in convoy to the factory gates, where they demanded entry.

Earlier in the afternoon, the officers had met in Preston police station, where they were shown aerial photographs of the factory before being “walked” through its building plans so that they would know what to expect when they raided the warehouse and where to position themselves to block escape routes.

Immigration officials prepare to enter a factory in Preston. Getting tough on immigration is prominent in coalition policy. Photograph: Gary Calton for the Observer

While the owners of Gafoor looked on, the officers secured the compound and started checking the identity of everyone inside. It was to be a busy night. Friday night is wages night at Gafoor and the factory was packed. On a key night up to 100 people are working inside.

The factory resembled something from a crime scene as immigration officers wearing face masks and white suits walked through the plant. Fingerprints were taken and requests made to the police national computer. Employees were asked their dates of birth, their mothers’ maiden names and their passport details. “It’s very difficult to fake that sort of stuff when you are put on the spot,” one immigration officer said.

Scenes such as these have been taking place up and down the country as immigration officers conduct high-profile raids designed to show that they are taking firm action against the suspected use of illegal labour.

Gafoor has been raided twice before, in 2008 and 2009, when it was under its previous management. On both occasions it was found to be using illegal labour and was fined £10,000 for each of the 33 workers that it used.

Those raids revealed that many of the illegal workers came from war-torn countries such as Iraq and Afghanistan.

But other, somewhat less obvious countries have a connection with the factory, too. Earlier this year the Observer interviewed two men from the Czech Republic who had been found on the streets by homelessness charity Thames Reach. The pair, one of whom had learning difficulties, claimed that they had worked at the plant and had been brought to the UK by a Roma gang from whom they had escaped.

In signed testimonies given to the police and shared with this newspaper, the men told how they were forced by the gang to surrender their passports and to work long hours at the factory. Their wages were taken from them and they were made to sleep five to a room in a house where they were ordered to perform menial tasks on their days off. They said they knew of others who had been trafficked into the country and forced to work in other factories.

The charity said it had come across 92 cases of people being trafficked into the country and then forced to work as modern-day slaves. “The trafficking of homeless males from across Europe and in the UK is on the increase,” said Megan Stewart, manager of Thames Reach’s London Reconnection Project. “The victims are being targeted by traffickers at London soup runs and day centres. It is vital that anyone offering a service to homeless people warn their clients of this very real risk.”

There is no suggestion that Gafoor, which is now under new management, knew the two Czech workers helped by Thames Reach were being exploited by a criminal gang or was in any way culpable for their treatment by the Roma gang.

By early Friday evening it was obvious to immigration officers trawling through Gafoor’s premises that their intelligence may have been correct and the company could be suspected of using foreign workers who had no right to be working in the UK.

If it has failed to carry out proper checks, it could be penalised for employing illegal immigrants, even if it had no knowledge of their immigration status. If the company has used the proper checking procedure, it will have a statutory excuse for failing to have spotted that these workers were illegal immigrants.

Three Indian employees were discovered hiding in a freezer unit and were promptly arrested. Two of them may have overstayed their visas and the third may be in breach of his visa conditions. Lancashire police are this weekend visiting a number of addresses in the area and Jon Cass, from the North West criminal investigation team, who led the operation, said he would go where the intelligence takes him.

“Illegal working fuels illegal immigration to the UK and that is why we are carrying out operations like these in Preston,” he said. “These arrests are a warning to anyone in the UK illegally. We will catch up with you and you will face arrest, detention and removal from the country.”

It is fighting talk, of a kind that will please a Tory-led government desperate to be seen to be getting tough on an issue that in private it sees as its achilles heel. The rise of Ukip has the government scrambling to convince the electorate that it is strong on immigration issues. Recently, the Tories’ election strategist, Lynton Crosby, addressed 180 Tory MPs at a retreat in the Cotswolds. His key message to the party? Get tough on immigration.

The issue is prominent in coalition policy. Last week the government published its immigration bill, which carries proposals for clamping down on illegal working and rogue employers. Under the proposals, the maximum penalty for those using illegal labour will be doubled to £20,000. The government is also planning to simplify right-to-work checks, in order to make it easier for employers to establish the immigration status of their employees.

“Illegal working encourages illegal immigration, undercuts legitimate businesses and is often associated with exploitation,” said immigration minister Mark Harper. “The existing illegal working regime isn’t simple enough; the penalties have remained the same since 2008 and don’t provide a sufficient deterrent; it is also too easy for companies to evade paying them.”

In 2010 the previous incarnation of the now clumsily titled Home Office Enforcement Immigration, the UK Border Agency, increased by almost a third the number of raids it carried out on factories suspected of employing illegal workers, focusing on the food sector.

“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 a reputation I’m very keen to change,” said Damian Green, the then immigration minister.

Last week the home secretary, Theresa May, used similar language when she outlined plans to create what she called a “hostile environment” for illegal migrants that will see greater restrictions imposed on their access to the UK’s privately rented housing sector, its banking system and its health service.

But the Crosby-inspired tough talking is in danger of colliding with reality. High-profile raids on food processing plants and other industries thought to employ illegal labour can take the government only so far in its battle to build the image of fortress Britain.

Beneath the bluster it has been shown that the UK’s borders are extremely porous. Last week it emerged that the eBorders scheme, a £500m system to stop criminals and terror suspects entering the UK, had failed to check a third of airline passengers entering the country. John Vine, the chief inspector of borders and immigration, reported that the scheme had stopped “not one person” from boarding a plane to the UK.

In the same week, the home affairs select committee published a report that found that some asylum seekers have been waiting 16 years for a decision on their case. A backlog of 32,600 asylum cases that should have been resolved two years ago had yet to be concluded, while the number of those still awaiting a decision after six months had risen by 63%.

Given these sorts of problems, it is hardly surprising that migrants end up working in the illegal labour market. Stuck in legal limbo, some believe they have little choice but to find jobs in places where few questions are asked about their employment status.

At Gafoor Poultry on Friday afternoon, the atmosphere was relaxed and co-operative. The company’s management said little as the immigration officers carried out their raid. The mood was cordial, almost bored. The management appeared to be confident that they had done nothing wrong.

Meanwhile their business is going from strength to strength. The company’s website boasts: “Our past has been inspirational but our future is full of promise.” This is certainly true. In August, the latest month for which figures are available, some 71m chickens were slaughtered in the UK, an increase of 3.1% on the same month in 2012. The UK’s appetite for poultry is insatiable. And the cheaper it is the better.

The company declined to answer questions from the Observer.