The mayor of London, Sadiq Khan, has pulled the plug on the controversial plan for a garden bridge across the Thames, announcing that he would not provide the vital financial guarantees needed for construction to begin.

In a letter to the Garden Bridge Trust, the charity leading the much-delayed project, Khan said he had taken the decision because of a continuing shortfall in fundraising for the scheme and the lack of the necessary land use agreements despite three years of talks.

The decision ends the long-running saga of the proposed construction of the bridge, which was championed by the actor Joanna Lumley, Boris Johnson and George Osborne, and drew anger from critics who saw it as a symbolic indulgence.

Planning permission for bridge, which was designed by Thomas Heatherwick and would have run from Temple on the north side of the Thames to the South Bank, expires in December, making the timetable appear impossible, Khan said.

“The funding gap is now at over £70m and it appears unlikely that the trust will succeed in raising the private funds required for the project,” he said.

“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.”

Sadiq Khan: ‘It appears unlikely the trust will succeed in raising the private funds required for the project.’ Photograph: PDN/SIPA/Rex/Shutterstock

Under the plans for the bridge, its future operational and maintenance costs are guaranteed from mayoral funds if the trust is unable to meet them through its commercial and fundraising activities.

Khan does not have the power to scrap the project, but his withdrawal of public guarantees effectively does so.

The proposal for a garden over the Thames, featuring 270 trees and thousands of other plants, was originally devised by Lumley and won support from Johnson, Khan’s predecessor as London mayor, and the then chancellor Osborne, who committed £60m of public money to the scheme.

The rest was intended to be raised from corporate donations, but fierce local opposition and fruitless talks with the housing trust occupying the south side of the project delayed the start of construction and costs rose.

Khan commissioned the Labour MP Margaret Hodge to investigate whether the bridge still represented value for public money. Her report, published earlier this month, recommended it be scrapped.

The bridge was likely to cost more than £200m, Hodge said, and £37.4m of public funds had already been spent without any building work taking place.

She also found the Garden Bridge Trust had lost major donors and secured only £69m in private pledges, leaving a gap of at least £70m. It has received no new pledges since August 2016.

The proposal featuring 270 trees and thousands of plants was supported by Boris Johnson and George Osborne. Photograph: Heatherwick Studio/PA

In his letter to Lord Davies, the Labour life peer and former government minister who chairs the trust, Khan said the cash had been spent before final land agreements had been sealed.

“It is concerning that a huge amount of effort and expenditure has been expended on other aspects of the project when there is a real possibility that agreement will not be reached before the expiry of planning permission, or at all.”

Noting the funding gap found by Hodge, Khan wrote: “Pledged funds being lower than two years ago strongly suggests that support for the project is not robust enough to generate the required funds.”

The trust’s accounts show “material uncertainty as to whether the trust can properly regard itself as a going concern”, he wrote.

Khan criticised the trust’s plan to start construction before all private funds were in place, saying it could leave London with “a partially built bridge which would either require completion or demolition” at taxpayers’ expense.

Saying he had begun in office as a supporter of the project, Khan concluded: “It does not seem reasonable to me to believe that all these obstacles can be overcome.”

The trust has the option of seeking to amend the various planning permissions to permit work to begin without public funding guarantees, but this would appear extremely unlikely.

Responding to Khan’s decision, Davies said the trust had received the letter with great regret.

“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,” 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.”