An investigation has revealed that migrants who pick vegetables for British supermarkets are treated like ‘slaves’ and forced to live in filthy conditions.

Desperate workers who move to Western Europe for the chance of higher wages are living in wooden huts and being denied basic hygiene facilities, a Channel 4 documentary discovered.

Instead, they are forced to squat in the very bushes next to where the vegetables, destined for shoppers’ salad bowls, are harvested.

They are not provided with soap or water to wash their hands, meaning diseases can easily be spread between the workers and make their way onto the produce. Some live in filthy shacks made of wood and plastic sheeting, not dissimilar to the makeshift tents seen in the ‘Jungle’ migrant camp in Calais.

But these workers are in southern Spain, providing salad vegetables for British supermarkets including Waitrose, Marks and Spencers and Sainsbury’s.

These same supermarkets tell consumers the food is produced ethically and that workers are treated fairly in the wake of the ‘Fair Trade’ boom.

But the workers have another story. They reveal that they are made to work around dangerous pesticides which cause some of them to fall ill and say it is common for them to be underpaid.

One vegetable picker who worked for a firm supplying courgettes to the UK said: “For the toilet you have to go in the bushes. What is there to clean yourself? For these courgettes to reach England, there are workers who have suffered a lot.”

But supermarkets will have a get out clause, saying the price war which all supermarkets have been fighting in order to compete with cut price chains such as Aldi and Lidl will have lead to this.

However, it has often been the case that supermarkets, with their huge power to ensure the survival – or failure – of farmers – have always dictated prices. Back in 1997 a Panorama programme revealed that suppliers would often get phone calls from major buyers informing them that they have to cut prices of agricultural products because they have a promotion on and need to maintain their profit margins.

One of the firms fingered by the investigation supplies salad and vegetables to Marks & Spencer, Waitrose, Sainsbury’s, Tesco and Asda is among those accused of using mistreated workers. The employers of this company say they have developed sinus conditions and bronchitis as a result of being sprayed with pesticides.

Sainsbury’s and Waitrose both responded to the evidence saying they would investigate immediately. And politicians have joined in the condemnation, saying the footage was “appalling” and showed “effective slave labour producing food in 2015”.

 Similar concerns were aired about Agroherni, a firm which supplies £22 million worth of salad, herbs and vegetables to Britain’s major supermarkets including Asda and Tescso. One worker said she had been left in agonising pain and needed multiple operations on her sinuses because of pesticides.

One lettuce picker reported being underpaid, saying, “If we work 26 days, they write down 16 or 18. They always steal seven or eight days.”

Tory Richard Drax, a former member of the Environment and Rural Affairs Select Committee, said: “If true, these allegations are appalling. It sounds like effectively slave labour producing food in 2015 which is utterly unacceptable.

“Supermarkets must explain themselves. It is up to them to know who is producing their food and in what conditions.”

And farmer turned politician Neil Parish said there was “always a price to pay for cheap imported food.”

Agroherni strongly denied that it mistreats or exploits agency workers and said the fair treatment and safety of workers was paramount. However they have since stopped using the employment agency and are instead going to employ workers directly.

In a statement from Sainsbury’s, the supermarket said they “Expect our suppliers to adhere to the highest quality and welfare standards, regardless of where they operate in the world. We are taking these allegations very seriously and will be conducting our own investigation.’

Waitrose confirmed their supplier is investigating the allegations.