London mayor Sadiq Khan has effectively pulled the plug on the controversial Garden Bridge, three weeks after a scathing report recommended it be scrapped.

Khan today wrote to Lord Mervyn Davies, the chair of the Garden Bridge Trust, informing him that the Greater London Assembly would not provide mayoral guarantees for the project.

The mayor had been assessing a report he commissioned into the Bridge, authored by Labour MP Margaret Hodge which blasted the procurement process.

In his letter, Khan said the continuation of the project would expose the London taxpayer to additional financial risk, both with regard to the bridge’s construction and its operation and maintenance.

In a swipe at his predecessor Boris Johnson – who declined to speak to Hodge as part of her inquiry into the Bridge – Khan said “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.”

Khan’s intervention effectively signals the end of the controversial project – which has been in development for five years – and the ambitions of Johnson, actor Joanna Lumley and designer Thomas Heatherwick.