The developer will retain its former joint venture partner Stanhope as development manager for the vast project.

Architect Rogers Stirk Harbour + Partners and engineer Arup drew up the plans for the site at the rear of the existing library building near St Pancras station in London.

It will involve a 100,000 sq ft extension to the British Library along with 600,000 sq ft of commercial space targeted at the life sciences sector.

Three contractors – Mace, Multiplex and Sir Robert McAlpine – are expected to bid for the project.

The extension plan includes complex basement engineering because the developer will also fund construction of a station box and supporting infrastructure for Crossrail 2 beneath the site should the London transport project ever proceed.

Cutaway of the proposed site showing the spaces dedicated to office and lab-enabled in relation to the existing British Library space

Proposed British Library extension will be built on a vacant site to the north of the existing Library building next to the Francis Crick Institute

Mitsui Fudosan’s announcement means that work to deliver the development, which has planning, will now be able to proceed as soon as possible, with the next stage comprising detailed design. 

Takeshi Iwama, Chief Executive of Mitsui Fudosan UK, said: “Our development commitment represents what we believe to be one of the largest single real estate investments into London by a Japanese company to date. 

“We have already acquired significant expertise in the life sciences sector across Japan and the US over almost a decade and will be bringing this to the British Library extension development, our first in this sector in Europe.”

David Camp, CEO of Stanhope, said: “Mitsui Fudosan’s decision to fund the British Library extension is both a major milestone for the project and a strong endorsement of the London property market. 

Visualisation of the British Library extension

“The 600,000 sq ft of commercial space will be flexible enough to suit a broad range of occupiers who will want to be located at the British Library, adjacent to King’s Cross St Pancras and The Francis Crick Institute and at the heart of the Knowledge Quarter. 

“This has been one of the most complex projects in pre-development, with many twists and turns over seven years since SMBL’s selection by the British Library. 

“Getting here has required patience, skills and teamwork between our team at Stanhope, Mitsui Fudosan, the British Library and neighbours such as The Francis Crick Institute as well as the local community and TfL.”