It was billed as a miraculous no-strings-attached gift to London, a fairytale forest on a bridge that would be entirely paid for by private sponsors. But this sparkling new “tiara on the head of our fabulous city”, as Joanna Lumley, cheerleader for the garden bridge, has called it, is turning out to be a rather costly crown. In a confidential letter obtained by the Guardian it has emerged that the public will be liable for the bridge’s annual £3.5m maintenance bill in perpetuity – on top of the £60m of public funds already committed to the project.

The news goes against everything London’s mayor has pledged to date. On Tuesday, Boris Johnson again denied that further public money would be lavished on the scheme. “The maintenance cost will not be borne by the public sector, I’ve made that clear,” he told LBC radio. Yet in a letter from one of his senior staff to the Garden Bridge Trust, he appears to have made other plans.

“The mayor has agreed in principle to provide such a guarantee … to secure the ongoing maintenance of the proposed bridge,” writes Fiona Fletch-Smith, director of development, enterprise and environment at the Greater London Authority, who adds that Johnson is “fully supportive” of the project. If the garden bridge can’t meet its own costs, Londoners will be forced to foot the bill.

The news was greeted with outrage by London Assembly members, as another example of reckless use of public transport funds. “It’s scandalous,” said John Biggs, Labour chair of the budget and performance committee, which recently published a critical report on the viability of London’s sponsored transport schemes. “Boris Johnson has been caught red-handed lying to Londoners. This fiasco, promising that maintenance costs won’t be borne by the public sector while at the same time drawing up plans to do exactly that, shows how little his word is worth.”

Facebook Twitter Pinterest ‘It will set hearts racing and calm troubled minds’ … or will it be a burden to the taxpayer for years to come? Image: Arup/EPA

It is the latest chapter in an ongoing saga that has seen Thomas Heatherwick’s £175m bridge over the Thames transform from a sponsored gift to a burden on the capital’s strained transport funds. In June 2013, Transport for London (TfL) commissioner Peter Hendy said that public money would cover no more than the “enabling costs” of £4m and that the bridge’s “construction and ongoing maintenance costs would be funded by third parties”.

In December 2013, the Treasury announced it would donate £30m towards the project, a figure then matched by TfL. In six months, the public contribution had gone from minimal to £60m. A year later, it has shot up to a potential £150m, with the maintenance costs capitalised to an additional £90m over the next 125 years.

“It’s outrageous to be spending this amount of the transport budget on something that is simply not a transport scheme,” said Biggs. “It’s a tourist attraction, in a place where there are already bridges ... London is crying out for crossings further east, as well as at Pimlico to Nine Elms. Boris’s focus has always been on glitzy vanity projects rather than what London actually needs.”

He compares it to the Barclays-sponsored cycle hire scheme, originally planned to come at no expense to the public, but which has since been found to cost the taxpayer £11m a year, making it the most heavily subsidised form of transport per user. “These schemes are always fast-tracked and justified without a full appraisal, because ‘they’re not going to cost us anything’,” said Biggs. “But there’s a track record of sponsored infrastructure not quite living up to its promise.”

The controversial bridge was hurriedly granted planning permission by both Lambeth and Westminster councils at the end of last year, after the Garden Bridge Trust said that work must be completed by 2018, to make way for construction traffic for the Thames Tideway Tunnel. In its planning conditions, Westminster insisted on a third-party guarantee to underwrite ongoing costs, out of fear of the council being lumbered with the bill for years to come, suggesting TfL could take the responsibility. When the assembly’s transport committee chair, Lib-Dem leader Caroline Pidgeon, asked the mayor if TfL would foot the bill, his answer was categorical. “I can confirm that no such agreement has been made,” he said. “Nor will I make any undertaking to do so.”

“It is concerning that only after the mayor has given planning permission to the scheme that he comes clean and admits a financial guarantee has also been provided,” said Pidgeon, on hearing the news on Wednesday. “He should have been clear months ago that far from being a private and charitable initiative, the garden bridge is actually a major drain on public funds. Everything about the funding and decision-making process relating to the proposed Garden Bridge has so far been as murky as the water that flows down the Thames.”

Facebook Twitter Pinterest ‘A tiara on the head of our fabulous city’ … But could it end up being a costly crown? Image: AFP/Getty Images

The mayor’s office was quick to stress that the guarantee is only intended as a last resort, if the Garden Bridge Trust fails to meet its annual fundraising target of £3.5m.

“The mayor is fully supportive of the approach that is being adopted by the Garden Bridge Trust and is of the view that their business plan is robust,” said a spokesperson. “However, in order to discharge the planning obligation imposed by Westminster, the mayor has agreed in principle to provide such a guarantee. The mayor is absolutely clear that the provision of such a guarantee does not replace the primary focus of the Garden Bridge Trust, which is to secure the upkeep of the bridge in perpetuity. The Mayor will be seeking the necessary assurances from the Trust that this will be achieved.”

But many have questioned the ability of the trust to meet its ambitious jackpot year after year. According to its draft business plan, seen by the Guardian, the organisation plans to raise the annual £3.5m target through a combination of corporate membership, temporary catering and souvenir stalls.

Companies can become a supporter of the bridge for £20,000 a year, which will allow them “to host a ‘Breakfast on the Bridge’ for entertaining”. A gala fundraiser and private hire of the bridge 12 times a year are hoped to raise a further £900,000, while the rest is planned to be covered by “pop-up type events”, inspired by concessions along the High Line in New York, and a “discreet range of merchandise, including T-shirts, stationery and bags” – which is optimistically projected to generate up to £200,000 a year.

“Their business plan is ludicrous,” said Michael Ball, director of the Waterloo Community Development Group, who has launched a judicial review against the planning permission. “Maybe they will attract enough private sponsors for the first year, but will all these people stump up money for the second year – let alone year 20 or 50? The public sector will have to step in very soon indeed. Either that, or they’ll be forced to start charging for tickets to cross the bridge.”

Given that they are expecting 7 million visitors a year, the total costs could be covered by a turnstile-charging 50p a pop. But the trust insists that “there is no intention to charge … and there will certainly not be any sort of ticketing system”. How long that policy lasts remains to be seen, particularly given that there will be a requirement for groups of eight or more to register in advance, in order to discourage protesters.

Elements of the bridge design, meanwhile, are turning out to lead strange double lives. The deck of the new raised podium where the bridge will land on the south bank is designed to handle queuing crowds of up to 2,500 at peak times, to ease pressure from the already crowded riverside walk. But the business plan also says the terrace will be rented out for events every weekend from May to October.

“With any other project of this scale, this stuff would be nailed down before planning permission was given,” said Ball, noting an agreement to take down the London Eye if it ever became unprofitable. But with the garden bridge there are no such conditions. The public is going to end up paying through the nose for a very long time.” £3.5m is an extraordinarily high upkeep cost, he added, given that the Hungerford Bridge’s two pedestrian crossings are maintained for £800,000 a year.

Questions have also been raised about the bridge’s origins, after the Architects’ Journal published letters from Lumley to Johnson, dating back to his election in May 2012, in which she talks of the plan for a bridge “to bring great loveliness to the Thames,” and signs off with “please say yes” and “thanks for the tulips”. In a BBC interview a year later, when asked how she has managed to push the bridge proposal so far, Lumley replied: “I’ve known Boris since he was four, so he’s largely quite amenable.”

“It’s far too cosy,” said Wai-King Chung of Thames Central Open Space, the campaign group that has launched a crowdfunding appeal to support the judicial review. “It’s Auntie Joanna asking little Boris for a new toy. The letter highlights the covert way the whole project has come about.”

A spotlight has now been focused on the procurement process, after it emerged that TfL issued a private invitation to tender in February 2013 to Heatherwick Studio, along with just two other firms – experienced bridge designers Wilkinson Eyre and London Eye architects Marks Barfield. The tender document asked for submissions of previous experience, but not a completed design. Miraculously, one team already had a worked-up scheme.

“The process is absurd,” said Ball. “For a major piece of infrastructure, the project should go out to OJEU (EU-regulated international tender), like the new Nine Elms to Pimlico bridge, and be a real international competition.” Peter Smith, a procurement expert and editor of the Spend Matters blog, has described the process as “sketchy”, adding that Heatherwick had a clear advantage ahead of the bid.

As more details emerge, former supporters of the bridge are jumping ship. The London Wildlife Trust has now joined the RSPB and Metropolitan Public Gardens Association in withdrawing its support. “The location and design of this bridge seems to reflect personal vanities rather than any meaningful attempt to connect Londoners to the capital’s rich natural and horticultural heritage,” said Carlo Laurenzi, chief executive of the trust, joining the RSPB’s claim that the bridge “falls short for wildlife”, by providing less than half a football pitch of green space. “This huge investment of public funds would be much better spent on improving communal green spaces across London,” added Laurenzi, “benefiting Londoners of all ages at a local and accessible level.”

But that would require a long-term commitment to improving the city, something for which our mayor has little concern. “Boris is like Macavity, the mystery cat,” says Biggs, referring to TS Eliot’s mastermind criminal feline, who is long-gone by the time the trouble arrives. “The bills will arrive after he’s safely left office – and he’s more than happy to leave a big IOU for whoever comes in next.”

