Salads in British supermarkets are being grown by workers treated like ‘slaves’ who live in filthy conditions, an investigation has found.

Migrant workers producing vegetables exported to Britain say they are denied basic hygiene facilities while working and are forced to use bushes near where the vegetables are being grown as toilets.

They are not provided with soap or water to wash their hands, while some live in filthy shacks made of wood and plastic sheeting near the fields in southern Spain.

They claim they are treated like slaves by agencies who hire them to produce the salads and vegetables that end up on supermarket shelves in Britain, the investigation by Channel 4 News found.

They also say they are made to work around dangerous pesticides – causing some to fall ill – and are routinely left unpaid for hours they have worked.

The revelations will horrify millions of customers of British supermarkets, all of which tell consumers their food is produced ethically and that workers are treated fairly.

And they will lead to claims that the bitter price war which has seen supermarkets slash prices in a bid to compete with discounters Aldi and Lidl has led to ruthless exploitation of the vulnerable.

A firm that supplies salad and vegetables to Marks & Spencer, Waitrose, Sainsbury’s, Tesco and Asda is among those accused of using mistreated workersSome workers say they have developed horrific sinus conditions and bronchitis as a result of being sprayed with the pesticides while producing the vegetables in the fields of Murcia.

Cleaning: Some live in filthy shacks made of wood and plastic sheeting near the fields in Almeria southern Spain

Yesterday British politicians demanded an urgent investigation, saying the evidence was ‘appalling’ and appeared to show ‘effectively slave labour producing food in 2015’.

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. Vegetable picker, Almeria

Sainsbury’s and Waitrose said they would investigate immediately. All the major British supermarkets claim they source food responsibly.

And they have all signed up to tough rules to promote workers’ rights under the global Ethical Trading Initiative, designed to protect labourers.

The revelations centre on workers in Murcia and Almeria from where millions of pounds worth of vegetables are exported to the UK every year. Workers in one field in Murcia were filmed being sprayed with pesticides.

One vegetable picker working for a firm in Almeria supplying courgettes to the UK told of the filthy conditions in which he was made to work.

‘For the toilet you have to go in the bushes,’ he said. ‘What is there to clean yourself? For these courgettes to reach England, there are workers who have suffered a lot.’

Growing trade: The workers say they have to work around dangerous pesticides - causing some to fall ill

He said workers at his company, based in Almeria, were also sprayed with pesticides but were too afraid to complain because their names would be put on a blacklist known as ‘the list of rotten sardines’.

Lettuce: The revelations will lead to claims that the bitter supermarket price war has led to ruthless exploitation of the vulnerable

The investigation also uncovered concerns about workers for Agroherni, a Murcia firm which sells £22million worth of salad, herbs and vegetables a year to Britain’s major supermarkets including Asda, Tesco, Sainsbury’s, Waitrose and Marks and Spencer.

It is not alleged that Agroherni's workers are forced to live in dirty conditions or work without sanitation, but one former employee told how she had been left in agonising pain and needed multiple operations on her sinuses after working in fields where pesticides were being sprayed.

The woman, who did not want to be named, said: ‘All that matters to them is fulfilling their clients’ orders.’

Agroherni uses an employment agency called Integra Empleo to provide workers to pick the produce in its fields, but the workers say the agency routinely denies them pay.

One lettuce picker said: ‘If we work 26 days, they write down 16 or 18. They always steal seven or eight days.’

Last night 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.’

Tory Neil Parish, a colleague on the committee, said: ‘There is always a price to pay for cheap imported food; whether it is poor quality, low animal welfare standards or, in this case, the appalling treatment of workers.’

More than 40 employment agencies supply labour to farm growers in the Murcia region.

All the top supermarkets say they abide by the Ethical Trading Initiative. Tesco says it has local ethical trade managers on the ground who investigate claims locally.

Bagged up: The revelations will horrify millions of customers of British supermarkets (file picture)

But it is thought the companies who are major suppliers to the supermarkets get round the rules by using employment agencies who cut corners to save money.

Both Agroherni and Integra Empleo deny all the allegations and say they have launched investigations into the claims.

Probe: Sainsbury’s (pictured) and Waitrose said they would investigate the findings immediately

Agroherni said it maintains a strict procedure with regard to the use of pesticides and is not aware of any relevant incidents.

It strongly denied that it mistreats or exploits agency workers and said the fair treatment and safety of workers was paramount.

However as a result of the Channel 4 News investigation it has ceased working with Integra Empleo and is to employ the workers directly.

A Sainsbury’s spokesman said: ‘We 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.’

A Waitrose spokesman said: ‘Worker welfare is very important to us – our supplier is investigating these allegations and will ensure that our high standards are being met.’

A spokesman for the British Retail Consortium which represents all the supermarkets said: ‘Ensuring workers are treated fairly in our supply chains is a key concern for retailers. We know all supermarkets will examine these allegations closely.’

Watch the full film tonight on Channel 4 News at 7pm.