A plan to build a bridge covered with trees and shrubs across the River Thames has lost the essential support of the mayor of London.
Sadiq Khan has written to the Garden Bridge Trust saying he cannot provide the financial guarantee needed for planning permission to be granted.
The letter follows a meeting between the trust and City Hall officials.
A review of the project said £37.4m had been spent and it would cost taxpayers £46.4m even if it was cancelled.
Dame Margaret Hodge, who carried out the review into whether the Garden Bridge offered taxpayers value for money, said the project should be scrapped.
'Risk to taxpayers'
Transport for London (TfL) had pledged £30m, but £20m of that was to be a loan, and the rest was from central government.
In his letter the mayor said: "Dame Margaret was right to conclude that the project progressing would expose the London taxpayer to additional financial risk, both with regard to the bridge's construction and its maintenance.
"I have been clear that this should not be allowed to happen. Accordingly, the GLA (Greater London Authority) is unable to provide mayoral guarantees for this project.
"I regret that the significant expenditure of public funds and effort from both public bodies and the Garden Bridge Trust has not led to a situation where I can provide the guarantees requested."
Mr Khan said the pledged funds the trust had detailed at the meeting on 20 April were lower than two years earlier "suggesting support for the project is not robust enough to generate the required funds".
Less than half the required private funds had so far been raised, he said.
Analysis by Tom Edwards, BBC London transport correspondent
The Garden Bridge's future is hanging by a thread - these financial guarantees are part of their planning permissions.
Crucially Sadiq Khan has not only withdrawn financial guarantees, he has withdrawn mayoral political support. That will make future private fundraising near impossible.
The taxpayer will lose £46m and questions over who is to blame for that will not stop. This mayor blames the previous mayor Boris Johnson.
The big problem with it has always been that it was never really a transport project and it's not clear why a bridge was needed there. It really is a tourist attraction.
The other problem - which was obvious back in 2012 when I first started covering this - was as a philanthropic gesture, those behind the project did not really ask the locals if they actually wanted it.
In January, the trust admitted its future was in doubt after publishing accounts, which showed a £70m shortfall in funding.
Dame Margaret's review criticised many of the decisions taken regarding the Garden Bridge saying they were driven more by electoral cycles than value for taxpayers' money.
Her review also found costs had risen from an early estimate of £60m to more than £200m.
The trust accused Dame Margaret of ignoring information in her review, which it claimed was full of errors.
In response to Mr Khan's announcement, the trust's chairman, Lord Davies, said it would be reviewing its options.
He accepted the trust faced a major challenge to find another organisation to provide the maintenance guarantee.
"The Garden Bridge Trust was set up at the request of Transport for London and the Department of Transport to deliver the project which had received public money," he said.
"We have had enormous support from our funders and are very confident we can raise the remaining funds required.
"But sadly the mayor of London has taken a different decision to those in place when the project started."