The British security company G4S has been condemned after winning a £70 million contract to service America’s Guantanamo Bay Naval Base, which contains the notorious prison for terrorism suspects.

As part of the contract the world’s largest security firm is to provide janitorial services to the Cuban prison, which currently houses 149 inmates who have not been charged with any offence.

Most of the firm’s responsibilities will involve “base operating services” at Guantanamo such as housing and facilities for soldiers and their families, according to a bulletin issued by the US Department of Defense. It will not be providing prison staff or operating cells.

Human rights groups have condemned the move. Kat Craig, the legal director of Reprieve, said: “Any company which associates itself with the legal black hole which is Guantanamo Bay has serious questions to answer. Guantanamo has been the site of detention without trial and brutal mistreatment for over a decade – even now, Reprieve’s clients are being subjected to torturous force-feedings in response to their peaceful protests.

“G4S must make clear exactly what the nature of its work at Guantanamo is, and given that the company receives millions of pounds of British taxpayers’ money, it has to be asked whether the UK Government – which has called for the closure of Guantanamo Bay – is comfortable with such an association.”

Rob Freer, Amnesty International’s USA researcher, added: “No-one, no government, no company, should be doing anything to perpetuate these unlawful detentions. All should be working to resolve them swiftly in a manner fully consistent with international human rights law and standards, and closing the Guantanamo detention facility once and for all.”

The $118m (£70.6m) contract was awarded to G4S Government Solutions, a US offshoot of G4S based in Florida which the company is in the process of selling off. A spokeswoman said the US part of the company operated as an “independent company” and that the British board of directors did not have any say over the business it accepted.