Indonesian labourers picking berries on a farm that supplies Marks & Spencer, Waitrose, Sainsbury’s and Tesco say they have been saddled with debts of up to £5,000 by unlicensed foreign brokers to work in Britain for a single season.

Pickers at the farm in Kent were initially given zero-hours contracts, and at least one was paid less than £300 a week after the cost of using a caravan was deducted, according to payslips and other documents seen as part of a Guardian investigation.

The fees they pay to secure work include flights and visas, but multiple labourers said they also faced thousands of pounds in extra charges from Indonesian brokers who promised substantial earnings. Under UK employment law, it is illegal to charge workers fees for finding them jobs.

One worker described how he staked his family home in Bali as surety on the debt and fears losing it. “Now I’m working hard only to pay back that money,” he said. “I cannot sleep sometimes. I have a family who need my support to eat and meanwhile, I think about the debt.”

Brexit and the war in Ukraine have created chronic labour shortages in the UK agricultural sector, pushing desperate farms and recruitment agencies to look further afield than Europe, where it can be harder to track the methods local brokers use to find workers.

The revelations raise the prospect of fruit pickers being trapped in debt bondage, preventing them from leaving work without risking financial ruin. Migrant rights experts say the situation puts workers at risk of what is essentially forced labour.

The Home Office and the Gangmasters and Labour Abuse Authority (GLAA) are looking into the allegations, and the supermarkets have launched an urgent investigation into the issues raised by the Guardian.

Hundreds of Indonesian farm workers have been recruited to work in Britain this summer on seasonal worker visas, the immigration route created to tackle a shortage of farm workers after Brexit.

Scores of pickers were sent to Clock House farm near Maidstone in Kent, which supplies berries to most major supermarkets and has appeared in an M&S advert.

Clock House said it was “deeply concerned” by the allegations and would “not have entered into an agreement with, or taken workers from, any entity that was involved in such activity [the charging of fees]”. It said it was working with the authorities to investigate the claims.

The Indonesian workers were supplied by AG Recruitment, one of four UK agencies licensed to recruit using seasonal worker visas. AG denied any wrongdoing and said it knew nothing about Indonesian brokers charging money.

AG was originally planning to recruit from Ukraine and Russia but changed its plans when war broke out in February, weeks before the picking season was due to start. Last year almost 20,000 Ukrainians came to Britain on seasonal worker visas, making up two-thirds of all those arriving through the scheme.

AG had no previous experience in Indonesia and sought help from Jakarta-based Al Zubara Manpower, who in turn went to brokers on other islands who charged exorbitant fees to the people they introduced, according to one Al Zubara agent.

Invoices seen by the Guardian show workers at Clock House with debts of between £4,400 and £5,000 to a broker in Bali who supplied workers to Al Zubara Manpower.

While flights and visa fees are included in these debts, other charges included compulsory language training not required by farms, and hundreds of pounds for accommodation in Jakarta while awaiting visas. The debts have to be repaid by bank transfer in monthly instalments of up to £800, according to documents seen by the Guardian.

Sukiasa Ketut, a freelance agent managing Al Zubara’s recruitment in Bali, said they used “many, many” freelance brokers who did not stick to the rules. He acknowledged that brokers pocketed money from charges and said he felt “really sorry” about the situation.

He added: “We didn’t prepare well regarding the rules of how to recruit people, so we [went] to the recruiters and they just did this in their own way. We were surprised as some charged more and others charged less.”

Even those workers who did not go through a third party were billed £2,500 by Al Zubara, with charges including a £560 training fee and a £362 visa fee, despite the Home Office listed fee being £259. Aside from £1,478 for the flight and the visa, there are not supposed to be unnecessary supplementary fees under the seasonal worker scheme.

The managing director of AG Recruitment, Douglas Amesz, said he was unaware of brokers charging money, and there is no suggestion that he knew of the fees. He said he personally recruited candidates in Jakarta and that applications and visas were only processed by AG.

Workers from Bali met Amesz in Jakarta and remember him telling them they should not pay any fees for jobs and that it was illegal. But they said local brokers told them not to disclose what they paid.

Clock House farm uses 1,200 workers on average every season to pick raspberries, strawberries, blackberries and plums.

It issued some Indonesian arrivals with a zero-hours contract, seen by the Guardian, despite this having been against the rules for those on seasonal worker visas since April 2022. After the Guardian approached it for comment in late July, the contracts were changed to guarantee a minimum of 20 hours.

Payslips seen by the Guardian show labourers sometimes worked far fewer hours and can make less than £300 a week, leaving them concerned about their ability to repay debts. Clock House said it conducted an audit of its payroll and found workers picked for an average of more than 48 hours a week, meaning an income of more than £2,000 a month.

Clock House pays workers £10.10 an hour, in line with the visa rules, and docks £10 a day from their pay to fund the caravans they live in on the site.

Lawyers representing Clock House said the farm was “deeply concerned” about the recruitment issues flagged by the Guardian “and the potential welfare and worry that it would place on seasonal workers” and has reported the allegations to the GLAA.

It said it was previously unaware of any problems, or of the existence of other recruiters aside from AG Recruitment. It has requested that AG supply no more workers from Indonesia until further investigation is carried out.

On working hours, Clock House said it was writing to all seasonal workers to confirm that at least 35 hours work a week would be guaranteed on average over a 28-day period.

Amesz said he was “extremely concerned to learn of the allegations that have been raised” and AG was “fully cooperating” with the GLAA. He said Al Zubara did not handle any recruitment and AG did not ask it to subcontract recruitment to other local organisations or brokers.

He added: “We continually monitor all aspects of our supply chain and if we find egregious behaviour anywhere, we are committed to change. In the light of your investigation, we will redouble our efforts to ensure that all partners and affiliates understand the law, understand their responsibilities, and understand our responsibilities under UK law and GLAA regulations in order to protect worker welfare.”

AG said the Indonesian ministry of labour had conducted an investigation and confirmed that Al Zubara had acted legally.

Leah Riley Brown, a sustainability policy adviser at the British Retail Consortium (BRC), said on behalf of supermarkets: “BRC members are committed to upholding high standards of welfare for all people who work in their supply chains, and are urgently investigating the allegations raised. The BRC continues to advocate for a single enforcement body to ensure that exploitation is prevented and worker’s rights are a top priority.”

Waitrose said it was concerned and would take the “necessary steps to make sure workers are treated fairly”. Sainsbury’s said workers’ safe and fair treatment was “extremely important” and it was working with government and industry “to introduce measures to prevent these practices from happening”. Tesco said it did not tolerate the use of illegal recruitment fees charged to workers and it was vital for any to be repaid in full.

The Home Office would not comment on specific allegations, but pointed out that the GLAA had no remit to investigate labour abuse and exploitation in other countries, saying it was incumbent on foreign governments to investigate.

Andy Hall, an independent migrant rights specialist who investigates issues of forced labour in supply chains in Asia, said: “It seems the GLAA, Home Office, [Foreign Office] and also UK government officials working in overseas UK embassies have no remit or authority to actually monitor or investigate potential or real recruitment abuses in the source countries of workers migrating to the UK for work.

“As migration of workers into the UK continues to take on increased importance, this is a significant black hole in terms of monitoring and regulation that needs to be urgently addressed if the government is to fulfil its global pledge to prevent forced labour.”

Hall said the challenges with the current agricultural recruitment model began with workers being made to bear the cost of flights and visas, which he said created significant debt before any other charges. “Workers often don’t understand what they’re getting themselves into. They’re at risk of falling victim to debt bondage, which is forced labour essentially, as they can’t easily then return home.”

The GLAA said: “While we will not provide a running commentary on specific investigations, we treat all allegations of labour exploitation extremely seriously and we would strongly encourage anyone with concerns to contact us. We will investigate and take appropriate action if workers are being exploited for their labour.

“The GLAA is not the lead agency for the seasonal worker visa scheme. We have accompanied UK Visas and Immigration on visits to farms, as we have extensive experience of inspecting accommodation and interviewing workers to check that their rights are being upheld.

“We have also been working directly with the four scheme operators this summer on a package of prevention measures aimed at educating workers about their rights in the UK, including the importance of not paying recruitment fees.”