Skanska will build a 16km dual carriageway between the Black Cat roundabout at St Neots and Caxton Gibbet.
The A428 between St Neots and Caxton Gibbet is the only stretch of single carriageway between the M1 near Milton Keynes and Cambridge.
Several junction improvements will be made, including a new three-tier junction at the Black Cat roundabout allowing free flowing traffic onto the A1 and the new carriageway.
It will also create better routes for walkers, cyclists and horse riders and deliver better connections to St Neots and its train station.
This major project will deliver significant benefits to road users and the local community, improving journeys between Milton Keynes, Bedford and Cambridge – helping to unlock economic growth in this fast-growing region.
The scheme is due to open for traffic in 2025-2026.