Costain, in partnership with Atkins, Halcrow and RPS Group, will design and construct an 8km section between Brynmawr and Gilwern.
The contract was let on the basis of early contractor involvement (ECI). This is where the contractor and its designer develop the preliminary design and, jointly with the Welsh Assembly, take it through the statutory orders process before completing the design and constructing the scheme if approved.
Work on site is scheduled to start in 2014.
Costain said that the project presents 鈥渟ignificant engineering and environmental challenge鈥� because the road passes through a number of sites of special scientific interest including the Clydach Gorge within the Brecon Beacons National Park.
Costain chief executive Andrew Wyllie said: "We are delighted to have been awarded this contract and are committed to deliver a high quality, safe and cost effective scheme for the Welsh Government and the local community."
Costain鈥檚 contract is part of a larger scheme to upgrade 40km of the A465 between Abergavenny and Hirwaun to dual two-lane standard by 2020.
The Abergavenny to Gilwern section is complete and was opened to traffic in May 2008.
A 拢75m ECI design and build contract for the Brynmawr to Tredegar section has already been awarded to Carillion last year and start of works is programmed in 2012.
The Tredegar to Dowlais Top section was completed in November 2004.
There is no programme date yet for the stretch between Dowlais Top and Hirwaun, but is planned for completion by 2020 in two sections.
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