Boris Johnson has controversially guaranteed that the estimated £3m annual running costs of a garden bridge over the Thames would be met by public money if private funds were insufficient.

The London mayor had promised that no more public money would be used on the project, following a £30m injection of cash by Transport for London. But Johnson has decided that the capital’s taxpayers will guarantee the future maintenance of the bridge – a decision that could mean considerable extra funding in future years. Critics have accused the mayor of acting without due public scrutiny or debate.

As recently as Tuesday, the Garden Bridge Trust announced that it had agreed to provide the guarantee to cover the cost of maintaining the structure, but documents seen by the Observer, signed by Johnson and dated only two days later on 4 June 2015, reveal that the mayor will act as guarantee to “cover the maintenance of the bridge structure and maintenance and operation of the garden and public space”, if the trust that is running the project cannot afford to do so.

In March Johnson told LBC radio: “The maintenance cost will not be borne by the public sector, I’ve made that clear.” The mayor has also told the London assembly, when questioned over underwriting the running costs of the project, that he would never “make any such undertaking”.

The documents calculate the annual costs of running the garden bridge at £2.8m, with critics estimating the bridge’s maintenance bill at £90m over its lifetime.

Last week, the London assembly voted in favour of withdrawing the £30m committed to the bridge by Johnson, saying that it wanted a full, independent audit of the procurement process.

Michael Ball of campaign group Thames Central Open Spaces accused Johnson of trying to bury the announcement that maintenance of the bridge should be guaranteed.

Ball said: “A few weeks ago, the mayor categorically stated that no more public money should go into the garden bridge. Then on a Friday afternoon he quietly slips out a ‘guarantee’, which could cost Londoners £90m or more to pay for the exorbitant running costs of this private vanity project long after he has gone.”

Ball added: “We will carefully study the reasoning and legality of this decision, with the intention of returning to the high court if necessary to challenge it.”

Caroline Pidgeon, leader of the Liberal Democrat London assembly group, said: “As recently as March, Boris Johnson proudly declared that the maintenance costs of the garden bridge would not be picked up by the taxpayer.

“The reality is now coming to light, and it is highly likely that the public purse will be raided year on year to pay for the high maintenance costs of the bridge. The latest revelation is just one further reason why the whole idea of the garden bridge, in its current location, must now be reconsidered.”

The new row comes amid increasing signs that the Garden Bridge Trust is running into financial difficulties. Despite the £30m contribution from TFL and another £30m from the Treasury, the trust has revealed that it still has a £47m shortfall, with construction work on the bridge due to commence at the start of next year.

This Wednesday, the Garden Bridge Trust will embark on its latest attempt to drum up financial support with a three-course dinner and “fundraising gala” within London department store Harrods. Guests will dine inside the store’s Georgian restaurant while being entertained by the singer-songwriter Jessie Ware.

The Garden Bridge Trust will hold a fundraising event at Harrods, which is owned by Qatar Holding. Photograph: Alamy

The venue and obvious need for money has prompted speculation among opponents that figures such as actress Joanna Lumley, whose idea the garden bridge was, are hoping to woo Qatar Holding, the investment arm of Qatar’s sovereign wealth fund, which owns the famous Knightsbridge store, into backing the project.

Meanwhile, the architect Lord Rogers has mounted a robust defence of the £175m project, describing the river crossing as a “jewel in a great world city”.

Writing in the Observer, Rogers, whose architectural partnership designed the Millennium Dome and the National Assembly of Wales, states that the “brilliant garden bridge will enhance our public realm and reconnect the city, strengthening London’s renaissance, celebrating the river, creating an oasis of calm and beauty, and opening up new perspectives on London”.

The Garden Bridge Trust said that it had so far raised £17m from individual philanthropists, £10m from companies including global bank Citigroup, mining company Glencore and financial services firm ICAP, alongside £40m from charitable trusts including the Weston Foundation and the Sackler Foundation.

A spokesman for the Garden Bridge Trust said: “For the bridge to have raised £67m in funding from the private sector, charitable trusts, and individuals in barely 18 months is an extraordinary achievement.

“It shows just how much support there is for this unique new green space and river crossing, open 365 days a year, and free to all who visit. We are in discussion with other potential funders and expect to make more announcements in the near future.”

A representative for the mayor said: “The reason for having a guarantee from the GLA is to meet a planning requirement imposed by Westminster city council. The mayor has made it clear that public money will not be used to cover maintenance costs of the bridge.”

The mayor’s office added that, given the trust’s projected revenue surplus, it was expected that the guarantee would not need to be called upon.