The Garden Bridge project in London should be scrapped at a total cost to taxpayers of £46m because allowing the project to continue risks even greater expense, a damning report by Dame Margaret Hodge has found.

The review, ordered by London Mayor Sadiq Khan, says the costs of the project are escalating, that the purpose of the bridge remains unclear, that the business case for the bridge is flimsy and that the procurement process in which Heatherwick Studio won the contract to build the bridge was “not open, fair or competitive”.

The report highlights uncertainty over the future of the Garden Bridge Trust, which has lost two major private donors and currently has a £70m shortfall in funding. Meanwhile, the projected cost of the bridge has soared from an initial estimate of £60m to more than £200m.

The bridge has already received £60m in public money, with the Department for Transport committing £30m, and Transport for London also giving the project £30m in the form of a loan repayable over 50 years.

When elected as Mayor of London, Mr Khan said no further public money should go to the project.

The bridge was initially planned to be funded by private backers, and further private funds are yet to be found to pay for maintenance of the bridge if it is built.

But Dame Margaret’s report found “the original ambition to fund the Garden Bridge solely through private finance has been abandoned. Furthermore the goalposts have moved several times and each time the risks to the taxpayer have intensified”.

She said: “The project has already used £37.4m of public money and the agreement to underwrite cancellation costs by the Government could bring the bill to the taxpayer up to £46.4m.

“I believe it is better for the taxpayer to accept the loss than to risk the additional demands if the project proceeds. In the present climate, with continuing pressures on public spending, it is difficult to justify further public investment in the Garden Bridge.”

The controversial project was originally proposed by Absolutely Fabulous actor Joanna Lumley, who first came up with an idea for a garden bridge in the late 1990s – suggesting a Diana Memorial Bridge.

In 2002, she lobbied then Mayor of London Ken Livingstone for a garden bridge.

But it was when Boris Johnson began his second term in City Hall that her idea began to take shape. Ms Lumley has previously said she has been friends with Mr Johnson “since he was four”. She said this made him “largely quite amenable”, to supporting the project.

Another acquaintance of Ms Lumley’s is the architect Thomas Heatherwick. According to her 2004 biography, Mr Heatherwick said he would be “happy to work on the bridge”.

In 2013, a procurement process for a new Thames crossing was opened, with three architects submitting designs for consideration.

Dame Margaret’s review points out that several meetings were had between Ms Lumley, Mr Heatherwick and Mr Johnson at City Hall and at Heatherwick Studio before the contract was awarded.

It also highlights a trip Mr Johnson and Mr Heatherwick made to Apple’s San Francisco offices in February 2013 in an unsuccessful attempt to lobby for money from the technology company.

Dame Margaret said: “The evidence leads me to believe that the procurement options were intentionally developed to enable Heatherwick Studio to qualify.”

In addition, she said that Heatherwick Studio was consulted on procurement options, while none of the other bidders were consulted in the same way.

One of the architects which submitted designs told Dame Margaret: “Now that the facts have slowly begun to emerge, it is clear that we were just there to make up the numbers and the outcome of the so-called competition had in reality already been pre-determined. We feel deeply embarrassed to have been used in this way by a publicly accountable body who should know better.”

A spokesman from City Hall said: “The Mayor wants to thank Dame Margaret Hodge for this thorough report, which raises some very serious questions about the way the project has been handled. The Mayor will now take some time to carefully consider it and its implications.

“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.

“The report also raises some specific concerns around how the previous Mayor influenced Transport for London practice on this project in areas like procurement. These were concerns that the current Mayor shared when he came into office and from day one he sought to address them.”

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