It ranks as one of the UK’s largest public contracts and includes old reactor sites at Hinkley, Sizewell and Dungeness.

The contract awarded to the Cavendish Fluor Partnership by the Nuclear Decommissioning Authority takes effect immediately.

It makes Cavendish Fluor Partnership the new parent organisation for the dozen Magnox sites around the UK.

The sum agreed upon at £4.2bn or around £311m a year is well below initial industry expectations of it being worth around £7bn.

The successful consortium saw off three rival bids from consortia lead by Bechtel/EnergySolutions; CH2M Hill; and Amec/Atkins back in May.

The consortium will build on the progress already made in decommissioning and environmental restoration at the Magnox sites, and is expected to deliver extra programme savings of £1.5bn.

Nine of the Magnox sites in Scotland, Wales and England are nuclear power stations that have stopped generating and are in various stages of decommissioning, while one, Wylfa on Anglesey, is still generating electricity but is expected to close by 2014.

Research sites at Harwell and Winfrith, which housed some of the UK’s earliest experimental reactors but are now well advanced in their decommissioning programmes, are also included.