As exclusively tipped in the Enquirer, the BM JV team beat rivals Dragados-Hochtief (DH JV) and BAM Nuttall, Ferrovial Construction (UK) and Vinci Construction Grands Projets (BFV JV) for the £2.3bn job.
The twin bored tunnel project will see some of the largest bored tunnels in the world at 16m wide.
At 4.2km long will also rank as the longest road tunnel in the UK.
The award of the central tunnel section is the last of the three major contract packages for the 23km route which will link Essex with Kent. Balfour Beatty will build the £1.2bn approach roads package north of the Thames and Skanska will build the £450m Kent roads section.