The programme will strengthen the electricity transmission network between Bedfordshire, Hertfordshire and North West London while creating enough capacity to connect five new data centres with a combined demand of 1GW.

Laing O’Rourke will deliver new and upgraded substations at Letchmore Heath, Elstree and St John’s Wood.

The Hochtief Murphy JV (HMJV) has been appointed to install new 400kV cables and upgrade the existing Elstree to St John’s Wood transmission tunnel.

Hyundai Electric will supply transformers and other major electrical equipment, while Siemens Energy will deliver shunt reactors at Elstree.

The project stretches from Sundon substation in Bedfordshire to Elstree in Hertfordshire and on to St John’s Wood in London.

It includes upgrades at Sundon, a 35km reconductoring programme on the Sundon to Elstree overhead line, an extension to Elstree substation, construction of the new Letchmore Heath substation and reinforcement works at St John’s Wood.

More than 200km of overhead line will be upgraded. Another 60km of new cable will be pulled through the existing 20km-long Elstree to St John’s Wood tunnel, which was designed to accommodate an additional transmission circuit when it was built.

National Grid said the investment will boost network resilience, unlock future connections and reduce long-term electricity constraint costs while supporting growing demand from homes, businesses and digital infrastructure across the capital.