The joint venture between Babcock and US firm Fluor was chosen by the Nuclear Decommissioning Authority (NDA). The 14 year contract covers twelve nuclear power sites, including Hinkley, Sizewell and Dungeness, and is estimated to be worth as much as £7 billion.
NDA chief executive John Clarke said the joint venture was chosen for its "successful track record and extensive nuclear experience".
The contract is one of the most valuable in the UK, and the bidding process took two years. Cavendish Fluor Partnership beat competition from three other groups to earn the contract.
Babcock chief executive Peter Rogers said the decision reflected its "ability to successfully deliver complex projects of national significance in very demanding environments".