The Welsh Assembly Government is seeking a contractor for the dualling of the A465 Heads of the Valleys road, from Gilwern to Brynmawr.
The ECI contract is worth between £140m and £180m.
The work involves development of the preliminary design that was presented to a public inquiry in 1998, and dualling of the existing 3-lane A465 Trunk Road between its junction with the A4077 at Glanbaiden Roundabout, Gilwern and its junction with the Section 3 works between Brynmawr and Tredegar, which will be under construction.
An integrated team including the design and build contractor, oads and Projects Division of WAG and the Assembly's agent E C Harris will work in partnership to carry the project through the development, statutory procedures, construction and aftercare phases.
The project is expected to run for 140 months.
The deadline for expressions of interest is 20 April.