Tower will be able to accommodate more than 6,000 workers with free public viewing gallery.

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A 56-storey tower called The Diamond is set to join the growing cluster of skyscrapers in the City of London and will be the financial district’s third-tallest building when completed.

The planned 263.4m tower at 100 Leadenhall Street will rank behind 1 Undershaft at 290m, nicknamed the Trellis, where work is yet to start, and 22 Bishopsgate, the reworked Pinnacle at 278m, which is under construction. All three will eclipse the nearby Heron Tower, currently the tallest building in the Square Mile at 231m.

The Diamond, previously dubbed “Cheesegrater 2”, will be built next to the 225m Cheesegrater once three smaller buildings have been knocked down.

Their tenants’ leases expire in 2023 but the developers will negotiate with them and hope to start construction sooner.

It is expected to take four years to build. The firm is working with the developer London & Oriental.