The Ethical Trading Initiative (ETI) is urgently advising UK food retailers to map their supply chains after it found “massive exploitation” of migrant workers involved in Italy’s tomato crop.

The crop’s supply chain was extremely vulnerable to mafia involvement, the report added.

Italian tomatoes account for more than 60 per cent of those sold in the UK and the ETI warned revelations of appalling conditions in the sector could damage the reputations of UK purchasers.

The ETI report Counteracting exploitation of migrant workers in Italian tomato production said that despite previous attempts at cleaning up the sector the migrant workforce in the Italian tomato sector was still controlled by illegal gangmasters.

These gangmasters, who are known as the “Caporali”, continue to hire migrant workers even though Italy banned the Caporalato system in 2011.

The report added that land hoarding, fraud, illegal hiring, labour exploitation and illegal transport meant the mafia “may intercept and colonise every step of value creation”. More than 25 per cent of all assets confiscated from mafia organisations are land, farms, and businesses connected to the agri-food sector, the report said.

This illegal control of much of the labour market could have knock-on effects on UK retailers who want to ensure their supply chains are abuse free, the ETI added.

Non-EU workers were disproportionately affected by the gangmaster system, typically working long hours while being paid 40 per cent less than the legal minimum wage.

“In areas most affected by issues of labour exploitation, living conditions can be dire, and many foreign seasonal labourers live in abandoned buildings or slums,” said the report.

UK retailers typically buy tomatoes from Italian processing plants, the ETI said. However, it added that UK retailers have the power and influence to act and should urgently map their supply chains, prioritising areas most at risk of exploiting migrant workers and assessing wages paid and hours worked.

The organisation said: “ETI advises that brands will also want to assess how their purchasing terms affect the situation and whether this is a driver of low standards.”

The organisation added that international retailers cannot be expected to take lone responsibility for changing this system and that Italian tomato processors and cooperatives must also examine their supply chains for abuse.

ETI also called on governments to provide increased legal protection for workers, to impose sanctions if these were recommended by the EU and to increase labour inspections.

While there are officially 116,000 migrant farm workers in Italy, unofficial figures suggest there may be as many as 500,000 in the country.