Foreign fruit pickers are taking home as little as £45 a week at a company which provides some of Britain's largest supermarkets with thousands of tonnes of fruit, an investigation by The Independent has found.

S&A Produce, which supplies both Tesco and Sainsbury's, employs thousands of eastern Europeans who are given a specific work visa allowing them to work for the company. They are attracted by the prospect of earning up to £200 a week by picking fruit on its farms in Herefordshire and Kent.

The workers are officially paid the minimum wage of £5.74, a comparatively high sum for foreign nationals who often have an average annual income of less than £3,000 in their own countries. But employee pay slips obtained by The Independent show that the real hourly rate for the company's fruit pickers often amounts to less than half the minimum wage once a series of obligatory charges has been deducted.

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One pay slip handed to The Independent by a Bulgarian employee who still picks strawberries on a farm in Brierley near Leominster, Herefordshire, showed that his net pay one week once the charges were removed was £45.12 for 19 hours of work – an equivalent of being paid just £2.37 an hour. Another employee at the same farm was paid £58 for 22 hours' work, the equivalent of just £2.61 per hour. The most substantial pay slip seen by The Independent, which was earned by a fruit picker last month, provided £70 for 16 hours' work, or £4.43 per hour.

At the peak of the fruit-picking season last month the firm hired more than 2,400 people, predominantly Romanians and Bulgarians who come to Britain on the Government's Seasonal Agricultural Workers Scheme (Saws), which allows them to stay on a single farm for six months. Unlike other eastern Europeans who have full working rights, Bulgarians and Romanians are unable to change their jobs if they fall foul of their employers or do not like their surroundings.

Each week S&A Produce, which is one of just nine operators of the Saws scheme appointed by the Government, deducts £31.22 for accommodation, which comprises either caravans or temporary "pods". The fruit pickers are also charged £2.75 a week for "entertainment", including internet access which some employees said rarely worked. Photographs of the computers handed to The Independent show that many have floppy disk drives, suggesting that they are at least seven years old.

The company also charges a "one-off" fee of £35 for "pastoral care", which includes helping employees make appointments with doctors or dentists and providing transport.

Employees interviewed by The Independent also said they thought they would be working for a full six months. But earlier this week S&A Produce, which is Britain's largest producer of strawberries, fired 346 employees who now have to return home, because the majority of the fruit picking had been completed.

There is no suggestion that S&A, which also uses the name S&A Davies, has broken any employment laws as the contract signed by the fruit pickers states that they will only be offered work when it is available. A spokesperson for the company said many fruit pickers had reduced hours this year because the weather conditions meant that much of the fruit was picked earlier than usual and that productivity was up 45 per cent on last year. But Peter McCaull, a councillor and former mayor of Leominster, said the firm needed to improve how it communicated to migrant workers the hours they would be working and how much money they could expect to make.

"If I had family that went out to work in Bulgaria I would expect them to be treated like human beings and be paid a fair wage for a fair day's work," he said. "These people have paid £200 of their own money to come over to work here and yet they are given barely enough work to survive. Many of them are unable to afford their flights back home."

Locals have complained that since the workers were laid off, some Bulgarians have been knocking on doors looking for cash-in-hand jobs to earn enough money to return home.

The revelations will also pile extra pressure on Tesco which was criticised last month by the Unite union for exploiting foreign agency workers in its UK meat and poultry supply chains. Last night a spokesperson for Britain's largest supermarket said it would investigate the claims. "We take the treatment of agency labour in our supply chain extremely seriously," the spokesperson said. "Our suppliers are subject to regular independent audits as well as our own checks. We are therefore very concerned by these reports and are working with S&A Davies to investigate them."

Last night Sainsbury's said it would also talk to the company. "We're meeting with our supplier to assess the situation and understand the basis of these allegations," a spokesperson said.

Rebecca Edmonds, a spokeswoman for S&A Davies, said the pay deductions were needed to provide accommodation and entertainment facilities and added that any accusation that the company was exploiting workers was unfounded. "We try to make it as clear as we can that we cannot guarantee the number of days or hours worked because of the seasonal nature of fruit picking," she said. "Regrettably we have had to offer fewer hours than we would have liked. But at least 84 per cent of the people we had to let go indicated in writing that they would like to come back next year which suggests that it is a minority, not a majority, or workers who have disliked working with us."