The London Garden Bridge Trust says the project will greatly benefit the capital
Boris Johnson has been accused of misleading Londoners over how much it could cost to fund a new garden bridge over the River Thames.
The mayor has promised no public money would go into maintaining the bridge, only £30m would go towards building it.
But Labour say a letter shows the mayor has agreed to pay the £3.5m annual costs if the trust behind the project fails to find private funding.
The mayor's office has not responded to claims he misled taxpayers.
Mr Johnson approved plans for the £175m bridge in December after Lambeth Council and Westminster City Council granted planning permission.
The chancellor and the mayor have pledged £30m each, and the rest is supposed to come from private funding.
Westminster council made one condition of granting planning consent that there was a clear financial guarantee that the running cost would be met.
On LBC radio this week, the mayor said his contribution to the new bridge would be "limited to a £30m contribution from TfL".
Pressed as to whether there would be any more public money used to fund the bridge he said: "No, the maintenance costs will not be borne by the public sector and I've also made that clear."
But a letter from Mr Johnson's officials to the trust 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."
In a statement, his office confirmed the content of the letter, and added: "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."
'Abusing the trust of Londoners'
Labour Assembly member John Biggs said: "Londoners all know, Boris Johnson knows, that in the end there is a risk of the public sector having to carry some of the cost.
"He has pretended that he has found some miracle way of this not happening.
"He is abusing the trust of Londoners in pretending it's not going to cost anything."
Last month, opponents launched a High Court appeal against the bridge, saying Lambeth Council unlawfully granted planning permission for the bridge.
Michael Ball, from Tulse Hill in Lambeth, said he feared the bridge would have a "devastating impact" for local residents as it would compromise the "best views of the City and St Paul's".
The trust said the bridge would also create new routes allowing for safer walking and improved pedestrian safety, while providing a new attraction for tourists.
