The contract involves construction of 4.5km of dual carriageway access road from the M4 Junction 38 into Port Talbot and the docks as well as completing the link to Junction 42 on the M4 and West Wales.
The scheme includes the construction of a bridge over the Swansea to London railway, railway realignment and signalling works and a new main entrance into Port Talbot Tata Steelworks (formerly Corus/British Steel).
Costain chief executive Andrew Wyllie said: "We are delighted to have been awarded this contract for Port Talbot which again underlines our position as a market leader in the construction and maintenance of highways in the UK."
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