Costain and Vinci have been awarded work on the £750M overhaul of the M4 in Wales.
The joint venture was appointed by the Welsh government to provide design development of the scheme around Newport.
Plans include 24km of new motorway and a 2.5km-long cable-stayed viaduct crossing of the River Usk, as well as major remodelling of M4 junctions 23 and 29.
The project will form part of the strategic roads network, and deliver government aims for a more efficient, safe and sustainable transport network.
Development work will commence immediately, with a Public Local Inquiry planned for winter 2016/7. Construction could start in spring 2018.
Costain chief executive Andrew Wyllie said: “We are delighted to be developing this critical infrastructure scheme for the Welsh government.
“This award is further demonstration of the importance to Costain of long-term strategic partnerships with our customers.”