LONDON — London Mayor Sadiq Khan withdrew his support Friday for a proposed garden bridge across the River Thames, saying City Hall would not provide the financial guarantees for construction and maintenance costs.

Khan's move means that it’s very unlikely that the tree-lined pedestrian crossing, an idea dreamed up by the British actress Joanna Lumley, will go forward.

There has been no shortage of hype about London’s garden bridge, which would create a floral linking of one area of the city to another — like the proposed 11th Street Bridge Park in Washington.

The designs by Thomas Heatherwick, a famous British designer, were drool-worthy, featuring a gorgeous crossing that linked London's Temple area, on the north side of the river, to the South Bank district. It would be a new natural space — boasting some 270 trees — with stunning views of the city skyline.

It could be, some argued, London's answer to the High Line in New York.

But at what cost?

As it turns out, building a luscious garden over a river in the British capital is not cheap — one estimate put the cost at more than 200 million pounds ($258 million).

The controversial project has became a flash point in debates over whether London is becoming a place only for the superwealthy. It requires taxpayer dollars to partly fund a project in an area already served by many bridges. At the same time, the garden bridge would be privately run, and organizers said that it would be closed off to the public periodically for corporate fundraising events.

The project received a further blow this month when an inquiry — set up by Khan — recommended scrapping it because it didn’t represent value for money. The author of the review, the Labour lawmaker Margaret Hodge, said that the costs for the project had spiraled beyond initial estimates and that there was a large funding gap that needed to be filled.

“What started life as a project costing an estimated £60m is likely to end up costing more than £200m,” she wrote.

When he was mayor of London, Boris Johnson was a big supporter of the garden bridge. His critics also accused him of championing a number of “vanity” projects.

When Khan became mayor, he offered tepid support for the project but said that no further taxpayer money would be sunk into it.

In a letter to the Garden Bridge Trust on Friday, Khan said that City Hall wouldn’t underwrite the project as there was a real risk that more public money would be needed to cover the costs of the project.

“The funding gap is now at over £70 million and it appears unlikely that the trust will succeed in raising the private funds required for the project,” he wrote. “I am simply not prepared to risk a situation where the taxpayer has to step in and contribute significant additional amounts to ensure the project is completed.”

Mervyn Davies, chairman of the Garden Bridge Trust, the charity that runs the garden project, responded saying that the organization had received Khan's letter "with great regret today" and that it would consider it before "responding formally."

“We have had enormous support from our funders and are very confident we can raise the remaining funds required,” he said. “But sadly, the mayor of London has taken a different decision to those in place when the project started.”

By Friday afternoon, the hashtag #GardenBridge was trending on social media in the United Kingdom, with many applauding Khan's decision and others expressing disappointment at the canceling of a unique urban design.