Britain's G4S has confirmed that it has sold its US subsidiary, which runs services for the controversial US naval base Guantanamo Bay, to an undisclosed buyer for $135m amid growing protest against the unit's existence.

The British security firm only secured the contract to run services for Guantanamo Bay, which currently holds 149 terrorism suspects without charge, three months ago.

Human rights charity Reprieve recently launched a formal complaint to the UK government's Department for Business, Innovation and Skills after G4S secured the US naval base's south-eastern Cuba contract for $117m (£74m, €93m).

"Any company that fully considers the human rights abuses committed at the Guantanamo Bay detention centre would find it impossible to justify accepting a contract that enables the continued detention and contributes to the suffering of detainees," said Reprieve's complaint.

Following the sale confirmation Reprieve said that G4S should still be under investigation for human rights abuses.

"It is crucial that the government investigates the support provided by G4S for Guantánamo, regardless of today's sale announcement. G4S must not be allowed to wriggle out of their responsibility for helping to run the legal black hole that is Guantánamo Bay," said Kevin Lo, a lawyer at Reprieve.

"The British government's stated policy is that Guantánamo should close. G4S, as the recipient of significant public funding, should not be allowed to undermine this policy without having to at least answer for it.

"Reprieve's goal in filing the complaint has not changed, and we continue to seek transparency from the company regarding the scope of the contract and how it may play a role in ongoing human rights abuses."

Meanwhile, G4S and the US defence department made it clear that the contract was for "operating support services", which includes a range of services from facility management to pest control.

The US defence department did not cite the actual detention centre in its list.