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.