Gangmasters raided by Kent police. Video: Felicity Lawrence and Richard Sprenger guardian.co.uk

There are strict rules covering the conditions for chickens and eggs marketed in the happy hen category by high street names – a Freedom Food chicken must be grabbed by both legs rather than just one when being caught, for example. But the conditions of the humans sent to catch them has had less attention.

Late on Monday a gangmaster company which provided teams of migrant workers to dozens of large chicken farms in a chain that supplies premium free range eggs to McDonald's, Tesco, Asda and M&S, "Woodland" eggs to Sainsbury's, and the Freedom Food and Happy Eggs brands to leading retailers had its licence revoked with immediate effect by the Gangmaster Licensing Authority (GLA).

The action followed a joint operation earlier this month by Kent police, the GLA and the Serious Organised Crime Agency to liberate more than 30 Lithuanian workers who are alleged to have been trafficked in to the UK. They are said to have then been kept in debt bondage, forced to work up to 17 hours a shift, bussed to farms the length of the country to catch hens through the night, sleeping for days at a time only in vans, in some weeks not paid at all, and, according to workers' testimony, kept under control by Lithuanian enforcers with threats of violence and on occasions actual physical assault.

A 52-year-old man and a 50-year-old woman from the company, who cannot be named for legal reasons, were arrested in a dawn raid on a Grade II listed house in Kent on suspicion of human trafficking for the purposes of labour exploitation. They were later released on bail, whose terms included a ban on them contacting the workers, pending further inquiries. The gangmaster company is a member of Freedom Food, the welfare scheme licensed by the RSPCA. Six dogs, including some fighting breeds, which the migrants claim were used to intimidate them, were taken from a separate property into the care of a local branch of the RSPCA.

The migrants, who as EU nationals are legally entitled to work in the UK, were transferred to a reception centre by the UK Human Trafficking Centre where those who wanted it were given support by the Salvation Army.

The gangmaster business supplied workers to Noble Foods, one of the UK's largest processors of eggs and chickens. Noble is currently promoting its happy eggs on primetime television with an advert that shows its hens leaping for joy to the Olympian soundtrack of Chariots of Fire.

The number of people being trafficked into the UK has been rising, according to a report published last week by the government's Inter-departmental Ministerial Group. Nearly a thousand suspected victims of human trafficking were referred to the National Referral mechanism in 2011, compared to just over 700 the previous year. Trafficking for labour exploitation is a significant and growing category, accounting for nearly a quarter of all cases. Extreme exploitation in the supply chain of major fast food names and leading supermarkets remains a serious problem, according to the GLA. Austerity across Europe, and retailers squeezing costs to counter inflation in food, have created the conditions for it to thrive. The GLA head of operations for the south-east, Neil Court, said exploitation among Lithuanians and other eastern Europeans appeared to be increasing, and in other cases the authority has seen a rise in organised crime, including fraud, money laundering, tax evasion and violence associated with labour abuse. The GLA has, however, had its budget for enforcement and inspection slashed by nearly one-fifth by the coalition government.

Court said the allegations in the case involving the chicken catching gangs were "probably some of the worst I have come across in my time at the GLA".

The workers said they had been charged a fee of around £350 for what they had been promised back in Lithuania were good jobs. £50 a week was then deducted from their wages each week, so they were debt bonded on arrival. They allege they would be told to bring food to last five days and were then bussed around the country from job to job, from Monday to Friday, sometimes being driven for five or six hours at a time between farms before working a night shift. They were put to work without training or safety equipment, having to learn on the job how to catch four chickens in each hand before crating them. Without face masks, they found the smell and dust in the sheds was often overwhelming. They described not even knowing where they were going, but trying to find out from the GPS system at the front of the minivan. They claim they were refused toilet stops on journeys and that in between jobs they were kept for hours in the vans at roadside parking places.

When they were brought back to their accommodation in Kent at weekends, they reported living 15 men to a small house in damp, squalid conditions. Mattresses on the floor were infested with bed bugs and fleas. They say they had £40 a week deducted from their wages for the privilege of being bitten as soon as the lights were out.

Those workers who wanted to open bank accounts or apply for national insurance numbers say they were told they couldn't and were threatened with the sack if they complained. When they were paid, it was by cheque, so they were forced to cash their wages at a local branch of the Money Shop where the charges are £2.50 for registration, a £3.99 flat fee for each cheque plus a 7.99% service fee, meaning that they could lose another £12 from a £100 cheque.

The most serious allegations from the workers relate to the climate of fear and violence in which they say they were kept. They have described Lithuanian enforcers used by the gangmasters to keep them under control with physical and verbal abuse. They have reported workers being beaten, punched, given black eyes and broken ribs, and then beaten again if they complained. One recalled first meeting one of the enforcers a couple of days after arriving in the UK, when the enforcer is said to have kicked the door of their house in and shouted to them that no one was getting paid that week. Others have said that their wages were withheld on several occasions for random reasons: a dirty cup left in the kitchen, or the smell of alcohol on someone's breath on a day off.

In revoking the company's licence to operate with immediate effect, the GLA listed breaches of licence terms on an unprecedented scale. The gangmaster has the right of appeal.

At the gangmasters' period property when the Guardian accompanied police on their raid, several luxury vehicles were parked in front of a green oak barn. A top-of-the-range motor home sat alongside two new Range Rovers. A decorative wooden shelter next to the gate was stocked with eggs for sale in free range and Freedom Food boxes with an "honesty box" in which passersby could post the money for them.

The labour-providing business supplied workers to Noble Foods, which contracts farmers to produce premium high welfare eggs which it then packs at a rate of 4m a day and supplies to McDonald's and the leading retailers. It specialises in brands and products which claim to meet high ethical standards. Its own brands include The Happy Egg Company, GoldenLay Omega 3 Free Range Eggs, Big and Fresh, Free Range Egg for Soldiers and Gu puddings. On its website it claims: "Concern for bird welfare runs throughout Noble's activities. The care with which we handle day old chicks is matched by our treatment of hens at the end of their laying life. Utilising a widespread Freedom Food approved national catching and collection network, our dedicated fleet of GPS tracked modular collection vehicles deliver directly into our abattoir and butchery operation."

Processors typically take responsibility for contracting labour to catch chickens for immunisation, for transferring to different sheds as they grow or for slaughter.

When asked what measures it took to ensure standards for workers as opposed to the hens in its supply chain, it said in a statement: "Noble Foods is one of many companies within the poultry industry that has used [this gangmaster]. After being notified of the action taken by Kent police we immediately ceased using this organisation. As the police investigation is ongoing it would be inappropriate for us to comment further."

The Guardian asked the gangmasters to comment on the allegations but they too declined because of the police investigation.

McDonald's also declined to comment and referred us to Noble Foods. A Tesco spokesperson said: "As a founder member of the Ethical Trading Initiative, we are committed to decent working conditions on all the farms and factories in our supply chain. These allegations are clearly shocking. We will continue to work closely with the GLA, our suppliers and others to ensure good practice throughout the sector." The other retailers also said that they took the allegations very seriously and were co-operating with the enforcement agencies.

The RSPCA's Freedom Food company said individual licence holders were responsible for ensuring animal welfare. There are no specific labour standards in its licence terms, but where a member does anything which would bring Freedom Food or the RSPCA into disrepute then the membership agreement makes provision for suspending or cancelling a business's membership. "Should these shocking allegations regarding workers prove to be true then Freedom Food would enact this provision," it said.