Costain's team will design and construct three 7m-diameter marine tunnels, around 11km in total length, to take in cooling water from the Severn Estuary for the nuclear reactor before it is cleansed, recycled and returned.
Now that go-ahead for the project has been agreed by EDF and the government, it is expected that work will start as soon as possible.
Costain chief executive Andrew Wyllie said: "We are delighted that Costain will play a key role in delivering the secure, low carbon energy the UK needs for the future through our role as a Tier One contractor in the Hinkley Point C project. The project will also help ensure Costain can play its part in building the skills of the UK and showcasing them to the world in a way which will help us to compete for other such contracts. We estimate that this contract will lead to the creation of hundreds of new jobs at Costain alone, including apprentices, specialist technical and engineering roles and programme management.”