Security guards at a Del Monte pineapple farm in Kenya that supplies most British supermarkets have been accused of brutally assaulting and killing people suspected of trespassing on its land.

A joint investigation by the Guardian and the Bureau of Investigative Journalism (TBIJ) has uncovered claims from villagers of violence by guards at the plantation in the last four years.

The vast plantation, which is estimated to cover at least 40 sq km, the area of a small city, is about an hour’s drive from the Kenyan capital, Nairobi. The farm has public roads going through it and directly employs 237 security guards.

Guards on the farm are typically armed with wooden clubs called rungus. Their use in security is legal and common in Kenya because of the risk of violent theft, including from young men who regularly go in organised groups to steal pineapples, but the claims suggest the guards’ use of violence has been excessive.

The scale of theft has resulted in clashes with guards, who have themselves been injured, including one reportedly losing an eye after a stone was thrown by a thief.

The claims of violence by Del Monte security guards over a number of years raise serious questions about the company’s processes and due diligence at the farm.

Del Monte said it took the allegations “extremely seriously” and has launched a “full and urgent” investigation into the claims. It said it is committed to international standards of human rights.

The farm, outside Thika, is the single largest exporter of Kenyan produce to the world, including the supply of tinned and fresh pineapple to supermarkets in the UK and elsewhere. Tesco, Waitrose, Sainsbury’s, Asda, Ocado and Morrisons are among the supermarkets that stock its fruit. Tesco said it had suspended orders on all products sourced from the farm until an investigation is concluded.

A law firm, Leigh Day, has written to Del Monte on behalf of 134 villagers living near the farm seeking compensation for “serious human rights abuses” allegedly carried out by security staff deployed on the pineapple plantation over the past decade.

The letter includes claims of five deaths, including the deaths of Karia, Nyoike and Wanginye, as well as five rapes, plus allegations of serious injuries, including head wounds, broken bones and cuts from blades requiring stitches since 2013.

Some alleged victims say they were indiscriminately attacked by guards while travelling on public roads through or near Del Monte land...