The contract is thought to be worth over £200m and ranks as BAM Construction’s largest project.

It is also a consolation for the BAM Kings Cross team, which had originally been earmarked to build Google’s 1m sq ft UK HQ at the regeneration scheme.

But this was lifted from the firm’s grasp after an eleventh-hour decision by Google to take the project back to the drawing board for a major redesign.

The buildings will span plot T2 and T3 on the northern part of the site and will become known as 11-21 Canal Reach when they are finished in early 2021.

Designed by architect Bennetts Associates the two curving Facebook buildings are expected to be rated BREEAM Outstanding offices.

The building on plot T2 will be 12 storeys high at its highest point at the southern end of the site, stepping down to nine storeys towards the north. There will be impressive roof terraces with views across the city.

BAM is onsite already with a substantial enabling and substructure works package before the main scheme commences in earnest.

Construction Director Ewen Hunter, who has led for BAM on several of the buildings at King’s Cross, said: “Nothing at King’s Cross is ordinary. Every building is ambitious and the client is constantly pushing what can be achieved for itself and others.

“Our teams embrace the challenge and our integrated model, involving our design and engineering businesses, makes us an ideal fit for the scale and complexity of this big building with a demanding design.”

The project is the last large building job in the transformation of King’s Cross, led by developer Argent through the King’s Cross Central Limited Partnership.

While three contractors were selected for the site-wide regeneration scheme, BAM has ended up building the lion’s share of projects compared to original fellow delivery partners Kier and Carillion.