The agreement limits TfL's contribution to the construction of the bridge to £10m rather than the original £30m promised. Any money spent in excess of the £10m will be treated as a loan and will be paid back over a 50-year term.
It is not clear whether the full £20m will be drawn down as a loan or on what interest terms. The decision means that the negotiations for the bridge's south landing site in Lambeth will resume after council leader Lib Peck originally called a halt to proceedings.
Labour mayoral hopeful Sadiq Khan, who previously opposed the bridge, reportedly now supports the project following the deal. The Garden Bridge Trust anticipates work beginning on site early in 2016. The National Audit Office has separately confirmed that it is looking at the rationale for the Department of Transport investing £30m of public funding in the project.