Featherstone’s new £150 million ‘superprison’ is to be sold off and privatised, Justice Secretary Ken Clarke announced this afternoon.

International security company G4S Plc will manage the prison when it opens in April 2012,  saving the taxpayer £31 million, Mr Clarke said.

Birmingham’s Winson Green Prison will also be run by the same firm from October 2012.

Doncaster Prison will also be sold off and managed by Serco Plc in the first of the new payments by result pilots. Buckley Hall in Greater Manchester will continue to be state-run.

Mr Clarke said the three new contracts would “secure significant improvements and savings at all the establishments involved”.

But unions, which oppose the privatisation, today warned of strike action.

Soldiers have already been given special training to take over the running of prisons in case of walkouts by prison staff.

The controversial Featherstone 2 was initially earmarked as a Titan prison with 2,500 inmates but was scaled back to house 1,600.