The construction company, which has already worked on the first stage of the project, has now been chosen to undertaken investigation works and to plan the refurbishment of the A40 Westway dual carriageway and the Marylebone flyover.
The transport link is used by over 96,000 vehicles entering and leaving the capital every day.
Marcus Hollan, project manager at Costain, said:
The A40 Westway is a vital transport link for Londoners. Tens of thousands of people use it every day to get to work, take their children to school or make deliveries.
This refurbishment will ensure that the Westway continues to keep Londoners moving in the decades to come.
The engineering firm's work will include undertaking a number of investigations along 21 structures along the 4.6km section of the route running through Kensington & Chelsea, Hammersmith & Fulham, and the City of Westminster.
These will assess the condition of key parts of the existing structures, while assessing out the potential scope for future refurbishment work. Costain will work with TfL to consult with key stakeholders to gain access to the structures such as Network Rail, as well as more than 120 local businesses occupying premises nearby.
The investigation works will take place in the summer.
After undertaking the feasibility study, which will take approximately 10 months to complete, Costain will then develop a plan for how best to deliver the refurbishment works.