The bridge, which is backed by actress Joanna Lumley, is due to receive £60m of public funding, including £30m from Transport for London which is chaired by Mr Johnson.
Taxpayers also looked to be in the frame for underwriting its £3.5m annual maintenance costs in perpetuity after Westminster council included this obligation in its planning consent.
The Mayor has repeatedly insisted he would not underwrite the costs, and in December told London Assembly Members: “I can confirm that no such agreement has been made, nor will I make any such undertaking to do so.”
Speaking on LBC radio earlier this week, he said Londoners’ contribution would be “limited to a £30m contribution from TfL”.
However the Mayor’s public comments appear to be contradicted by a letter from his office to the trust responsible for planning and building the bridge.
The letter, which was obtained by Labour Assembly member John Biggs, says: “The Mayor is fully supportive of the approach that is being adopted by the Garden Bridge Trust and is fully confident that your business plan is robust.
“However, in order to discharge the guarantee requirement imposed by Westminster, the Mayor has agreed in principle to provide such a guarantee.”
Commenting on the letter, Mr Johnson’s office said: “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.”
Some politicians and local campaigners have expressed doubt that the trust will be able to raise enough money to keep the bridge in good order without introducing admission fees or asking for more taxpayer money.
Mr Biggs said: “Boris Johnson has been caught red handed misleading Londoners, promising that maintenance costs won’t be borne by the public sector whilst at the same time drawing up plans to do exactly that. This fiasco shows how little Boris Johnson’s word is worth.
“In East London, where we desperately need new river crossings, the Mayor expects people to pay a toll. For the Garden Bridge, a scheme which is little more than a tourist attraction, he is willing for taxpayers to underwrite millions of pounds each year in costs.
“Boris’ focus has always been on glitzy vanity projects rather than what London actually needs. Underwriting millions for the Garden Bridge shows just how much taxpayer money he is willing to risk to keep Joanna Lumley happy.”
The news that Londoners will underwrite the bridge’s maintenance costs despite earlier promises has also been criticised by City Hall’s Liberal Democrat group.
Caroline Pidgeon AM commented: “It is concerning that only after the Mayor has given planning permission to the scheme and having allocated £30 million of public funds that he finally comes clean and admits that a financial guarantee has also been provided.
“The Mayor should have been clear many months ago that far from the Garden Bridge being a private and charitable initiative it 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.”
