The protected view of the Tower of London from The Queen's Walk outside City Hall will be one of the key issues at a public inquiry into plans for The Tulip skyscraper in the Square Mile.
Mayor of London Sadiq Khan is gearing up for a legal battle to block the London landmark that would be the capital's second tallest building.
The Mayor has earmarked £350,000 to fight an appeal against his decision last year to block the Tulip.
The skyscraper, named for the shape of its distinctive viewing gallery, would be a tourist attraction offering views of central London from the heart of the financial district.
Architects Foster + Partners got the green light for the scheme from the City of London Corporation last April.
Work on the 305-metre attraction was due to start this year, with plans to open in 2025.
But in July, the Mayor of London blocked the project on the grounds of poor design, and limited public access.
A City Hall review panel also concluded that the new build would damage views of the Tower of London, a UNESCO World Heritage Site.
It is the Tulip's impact on this protected view that will be hotly debated at the public inquiry.
Historic Royal Palaces warned that the Tulip "would diminish the Tower world heritage site, reducing it to the appearance of a toy castle, set down between the ever-growing Eastern Cluster [of Square Mile skyscrapers]... and Tower Bridge".
Now the Tulip's designers have lodged an appeal with the the Government, calling on housing, communities and local government secretary Robert Jenrick to reverse the Mayor's decision.
But Mr Khan is ready for a fight – and has put aside £350,000 to counter the appeal.
He has also written to Mr Jenrick urging him to refuse the plans.
In his letter, the Mayor claimed the Tulip would "cause harm to the historic environment (of the Tower of London), the wider skyline and image of London".
A spokesperson for the Mayor admitted that the potential legal spend was higher than normal, because the appeal is expected to take longer than in most cases.
He said: "The Mayor is clear that this proposal would be entirely inappropriate and he makes no apologies for standing by his decision.
"Any expenditure will be a result of the developer not respecting a legitimate planning decision made by the Mayor, which was supported by professional planning advice and Historic England."
He said the Mayor was not opposed to tall buildings in principal "when located in the right place" – but claimed the Tulip would "make little or no economic nor social contribution to the capital".
Foster + Partners could not be reached for comment.
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