The tunnelling job forms part of National Grid’s £600m investment plans for the capital to modernise the electricity transmission network.
Work will start on the 4m diameter high voltage electricity cable tunnels later this year. The first stretch will run between Hackney and Willesden in the north followed shortly after by another connecting tunnel running from Kensal Green to Wimbledon.
The tunnelling project is expected to take four years to complete.
Andrew Wyllie, chief executive of Costain, said: “This major award demonstrates Costain’s ability to provide cost-effective engineering and technology-led solutions in this vital sector.
“Costain also welcomes the opportunity to develop a long-term partnership with National Grid.”
London accounts for 20% of the UK’s electricity demand and this is growing by 3-5% a year, outstripping the national average at around 1%.
To meet this demand National Grid plans to build deep tunnels to house new 400,000 volt cables connecting existing substations.
Another cable tunnel running over 7km from Hurst to Eltham in south east London will also be built. Eventually National Grid will extend this section a further 10km to New Cross.