The future of the Garden Bridge is now in severe doubt after Mayor of London Sadiq Khan declined to provide maintenance funding guarantees the project requires before work can start.
Mr Khan said on Friday: "Under the previous mayor, a considerable amount of London taxpayers' money has already been spent on the Garden Bridge. I have always been clear that not a penny more of taxpayers' money should be allocated to the project.
"Having assessed all the information available to me including the findings of Dame Margaret Hodge's independent review, my view is that providing mayoral guarantees will expose the London taxpayer to too much additional financial risk.
"With planning permission due to expire this year, many outstanding issues remain, including spiralling construction costs and doubts around funding the maintenance of the bridge.
"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. 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."
Until now Mr Khan has repeatedly insisted that "the taxpayer will be better off if the bridge is built".
As recently as December – whilst Dame Margaret's investigation was under way – the mayor told the Garden Bridge Trust that he did not believe that the probe would reach the conclusion that cancelling the project was in taxpayers' best interests.
Mr Khan's change of heart is a major victory for anti-Garden Bridge campaigners led by the Thames Central Open Spaces group and backed by Kate Hoey MP and Bishop's ward councillors.
The mayor's decision is likely to prove fatal to the project, although in theory the necessary funding guarantees – a condition of planning permission – could be provided by another institution or company.
Lord Davies, chairman of the Garden Bridge Trust, said: "We received the mayor's letter with great regret today.
"We will study the contents of the letter in detail before responding formally.
"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.
"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."
If the project is to proceed, the Garden Bridge Trust also needs to reach agreement with Coin Street Community Builders, leaseholders of land on the South Bank required for the bridge's construction.
Coin Street chief executive Iain Tuckett said: "Coin Street Community Builders has always said that decisions on how public money is allocated and whether the Garden Bridge should go ahead should be made by the relevant public authorities.
"Sadiq Khan inherited a situation where a large amount of public money had already been invested in this scheme and we understand the reluctance to see that money wasted. However we welcome the mayor's decision to end uncertainty and ensure that further time and money is not expended."
Earlier this month the Garden Bridge Trust launched a stinging attack on Dame Margaret's report, branding it "one-sided".
Lib Dem London Assembly member Caroline Pidgeon said: "The Mayor has taken a year to make this decision and while he has been dithering more money has been wasted.
"Yet while it is regrettable that Sadiq Khan has taken so long to make a decision he has finally done the right thing following Dame Margaret Hodge's damning report.
"it is quite clear this vanity project of Boris Johnson's would end up as a bottomless pit for the public purse.
"It is now dead in the water and will be held up for many years as a case study on wasting public money and poor decision making at both government and mayoral level."
Labour's Tom Copley AM said: "How this project got as far as it has done is beyond belief. From the flawed procurement process, to allegations of conflict of interest, to extremely poor value for money, the Garden Bridge project has been disastrous every step of the way.
"Millions of pounds of public money has been wasted on what is essentially Boris Johnson's vanity project, despite a range of objections from a number of London Assembly Members. I am absolutely delighted that Mayor Khan is putting an end to this piece of Johnsonian indulgence and placing the interests of Londoners and taxpayers first.
"I'd also like to pay tribute to campaigners from the local community around the South Bank who have fought tirelessly against this bridge, and to the investigative journalism of the Architects' Journal."