The deal covering a dozen sites is expected to be worth around £6-7n over 14 years. It ranks as one of the UK’s largest public contracts and has attracted a heavyweight line-up of bidders.

The invitation for final bids ends a nine-month information-gathering phase, where bidders were able to visit Magnox and Research Site Restoration Limited sites.

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.

The two RSRL sites at Harwell and Winfrith are former research centres that housed some of the UK’s earliest experimental reactors but are now well advanced in their decommissioning programmes. RSRL is currently owned by Babcock International Group.

The winning bidder will take ownership of shares in the two Site Licence Companies and introduce a new senior management team to oversee decommissioning activities.

A preferred bidder will be named next March, ahead of the existing site workforces transferring over in September.

Andrea Livesey, the NDA’s Competition Programme Manager, said: “The formal dialogue period has concluded successfully, after a huge amount of time and effort by all parties. This really formed the crux of the competition process, enabling the participants to understand our requirements and secure the information needed to put their proposals together.”