Costain has been awarded a £200m contract from National Grid to construct its London Cable Replacement Tunnel.
The work forms part of National Grid's upgrade of the capital's electricity transmission network.
Costain will build a new high voltage electricity cable tunnel between Hackney and Willesden, via Kensal Green, and from Kensal Green to Wimbledon.
The tunnel will be 33km long in total.
Construction will start later this year, with the tunnelling being completed by the end of 2014, with all associated works completed in November 2018.
Andrew Wyllie, chief executive, 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.”