The contract, which covers the last phase of the mainline electrification, follows a four-month Early Contractor Involvement phase to nail the scope, programme and cost.

The project has been signed off after Network Rail admitted 18 months ago that its ambitious London to Cardiff electrification plans would cost £2.5bn-£2.8bn instead of the £874m projected in 2013.

Since then Network Rail has collaborated with contractors to work up precise engineering cost estimates.

Balfour Beatty will start work this month on the route section 6P, 8 and 9 between Cardiff and Bristol Parkway, using the latest technology and innovations in design, construction and rail plant to drive efficiencies and improve safety.

During peak construction, the project will provide employment for over 300 people.

Mark Bullock, Managing Director of Balfour Beatty’s UK Rail business, said: “Our selection to deliver this important upgrade to the UK rail network builds on our partnership with Network Rail and recognises Balfour Beatty’s capability and expertise.

“Balfour Beatty and Network Rail have successfully delivered the Crossrail West Outer Electrification Programme to schedule and I look forward to passengers seeing the benefits which will come when this new section of electrification is complete.”

Main works are scheduled for completion expected towards the end of 2018.