The Corporation was approached by owners of the site at 22 Bishopsgate for help overcoming opposition from nearby building owners concerned their right to light would be blocked, that threatened to delay the development.
The Planning and Transportation Committee voted through a proposal for the corporation to temporarily acquire the site on Tuesday, under a section of law allowing it to overrule "right to light" claims if those making them receive compensation.
A local authority can make use of these powers - under Section 237 of the Town and Country Planning Act - to "assist delivery of developments which achieve public benefit".
A report considered by the committee found the scheme would provide "a significant increase in flexible office accommodation and supporting the strategic objective of the corporation to promote the city as the leading international financial and business centre".
It said: "It is the view of officers that, given that negotiations which have been undertaken with affected owners, and, given that there are a large number of interests where agreement has not been reached, the conclusion to be reached is that absent engagement of Section 237, the development is unlikely to proceed, and certainly will not proceed within the timescale contemplated."
The tower is set to be built on the site of the former Pinnacle development, which was granted planning permission in April 2006. Construction began but stopped four years ago when funds dried up , and in November 2015 t he city resolved to grant planning permission for the site's redevelopment
The 62-storey, 912ft (278m) building will aim to provide 1.4 million sq ft of net usable space. A free public viewing gallery will crown the tower, and shops and services including restaurants, doctors and dry cleaners will be at the bottom.
Developers hope to provide space for around 12,000 people and up to 100 companies, and it is estimated to open for business in 2019.
A City of London spokesman stressed that section 237 powers were used "sparingly" and only "with careful consideration and after receiving the appropriate legal advice which we have done in this instance".
They were used previously to allow the go-ahead of the Walkie Talkie building (20 Fenchurch Street), also in the city.
He added: "In this instance, the view of the committee was that their decision was justified and this is a major development which will result in significant improvements to the local area, increased floor space and help create jobs."
The plans must be given the final go-ahead by London Mayor Boris Johnson.
Law firm Eversheds, which represents the affected owners, declined to comment.
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