A project to build a bridge covered with trees and shrubs across the Thames in London should be scrapped, a review has found.
Dame Margaret Hodge's review said it would be better to ditch the Garden Bridge than risk uncertain costs.
Three months ago the Garden Bridge Trust admitted its future was in doubt after publishing accounts which showed a £70m shortfall in funding.
Initially £60m of public money was pledged on planning for the bridge.
Transport for London pledged £30m, but £20m of that was to be a loan, and the rest was from central government.
Dame Margaret says £37.4m had already been spent, and even if the bridge did not go ahead it would cost the taxpayer £46.4m.
The Garden Bridge Trust said it was seeking a meeting with the London mayor to discuss next steps, but added the trust was "determined" to make the bridge happen.
Dame Margaret, the MP for Barking and Dagenham and former chair of the House of Commons Public Accounts Committee was asked to examine the project by London Mayor, Sadiq Khan in September.
On Friday, Dame Margaret said: "Value for money for the taxpayer has not been secured.
"It would be better for the taxpayer to accept the financial loss of cancelling the project than to risk the potential uncertain additional costs to the public purse if the project proceeds."
Dame Margaret said decisions on the Garden Bridge were driven more by electoral cycles than value for taxpayers' money.
Her review also found costs spiralling out of control from an early estimate of £60m to over £200m now.
Moreover, the review found the "risk to the taxpayer has intensified", while the original ambition to "fund the Garden Bridge through private finance has been abandoned".
Other findings include:
- The Trust has lost two major private donors and has had no new pledges since August 2016
- Very little progress has been made on raising money to fund maintenance of a completed bridge
- There was not an open, fair and competitive process around two procurements
Dame Margaret's review criticised the tendering process for the design of the Garden Bridge created by the previous City Hall administration.
It stated architects the Heatherwick Studio were given a head start in the process having already been closely associated with the project for five months before invitations to tender were sent out.
Two other firms that took part in the tendering process - Wilkinson Eyre and Marks Barfield - were given just eight working days to submit designs, the review said.
David Marks, co-director of Marks Barfield, told the BBC: "This competition didn't smell right from the start. It is clear that we were just there to make up the numbers and the outcome of this tender had in reality already been pre-determined.
"Had Marks Barfield been aware of Heatherwick Studios' involvement and original idea, we would certainly not have submitted a tender.
"We've never been asked to do something in such a short space of time."
Analysis: Tom Edwards, transport correspondent:
This is 40 pages of brutal, uncompromising criticism of nearly every part of the once flagship Garden Bridge project. I can't remember reading a report so damning of a transport project.
Procurement, finances, business cases, value for money, previous mayors, deputy mayors and transport officials get an absolutely withering assessment.
This was meant to be an oasis of green calm hanging above the Thames.
Now a report has basically recommended it be cancelled and the taxpayer take a hit on a project whose overall cost is now upwards of £200m.
It will be very difficult for the project to withstand this.
The big question now is will the present Mayor Sadiq Khan just let it wither or kill it?
Lord Mervyn Davies, chairman of the Garden Bridge Trust, said: "We will be studying the report in detail and seeking a meeting with the mayor to discuss next steps.
"The Trust remains as determined as ever to make the Garden Bridge happen which will bring huge benefits to London and the UK."
A City Hall spokesman said: "The Mayor has been absolutely clear that he will not spend any more of London taxpayers' funds on the Garden Bridge.
"It is the Garden Bridge Trust that remains responsible for raising the necessary funds, and delivering the project."