Controversial British security firm G4S has been accused of “stonewalling” over concerns regarding its involvement in the running of the US gulag at Guantanamo Bay.

Human rights charity Reprieve submitted a complaint to the British government earlier this year, arguing that G4S has breached Organisation for Economic Co-operation and Development (OECD) guidelines with its contract to supply “base support operating services” at Guantanamo.

The nature of the services provided by G4S are not made clear, and the company has allegedly refused to give further details about the $113 million (£71m) contract, prompting concerns that G4S could be involved with the abuse of detainees at the prison.

148 men remain held at Guantanamo, including British resident Shaker Aamer.

Mr Aamer has twice been cleared for release from Guantanamo, where he says he has suffered appalling torture and abuse, yet he remains detained without charge or trial.

G4S, a long-standing contractor for the British government, has been embroiled in a number of controversies in recent years including repeated allegations of the use of excessive force by employees during deportation removals.

Kevin Lo, an investigator at Reprieve, said: “G4S is deeply involved in running the legal black hole that is Guantanamo Bay, and the British public deserves better than the company’s stonewalling of questions. For the UK government to show that its condemnation of Guantanamo is more than just talk, the UK national contact point needs to press G4S for accountability and transparency.”