The Chancellor put £30m from the Department of Transport's budget toward the plan and urged Mayor Boris Johnson to match fund £30m from TfL. TfL is responsible for administering the total £60m of public funding for the project.
NAO press officer Rupi Gohlar told HortWeek the NAO intends to look at the "rationale" for the Department for Transport contributing £30m towards the Bridge, as well as looking at the "level of assurance they get from TfL for this funding". Any concerns will be raised with the Public Accounts Committee chair.
However Gohlar added the NAO would not be investigating TfL's procurement process or the value-for-money case for the investment, as the London Assembly was best placed to scrutinise funding and procurement decisions.
TfL has come under scrutiny after questions were raised over the procurement process which saw designer Thomas Heatherwick chosen over candidates with cheaper bids and more bridge design experience.
A published report into the internal audit at TfL found no fault with the process, but a draft version leaked to the Architect's Journal and the Greater London Authority was more critical. TfL's director of internal audits, Clive Walker, was questioned over the report in a meeting with the GLA Oversight Committee on 22 October, and agreed with members' assertions that the process had not been open or objective.
The move adds to concerns about the viability of the bridge, with Lambeth Council leader Lib Peck also backtracking on supporting the bridge, despite signing off planning permission in 2014. The Garden Bridge Trust still requires signoff from Lambeth Council to lease the bridge's southern landing site land. Peck cited concern about the public funding of the bridge at a time of public spending cuts as a reason for her change of heart.
The London Evening Standard has reported that the Trust could be willing to return some public funds; the trust could not confirm this to HortWeek.
"The Garden Bridge Trust has been very clear from the outset that the public funding received is a way to stimulate private contributions, which will make up the greater amount of the funding," a trust spokesperson said. "£145m has been raised overall towards the project so far, which is £60m of public funding and £85m of private funding."
Since planning permission was granted £20m of that public funding has been spent on preconstruction work. Major sponsor announcements are also in the pipeline, the spokesperson added.