The Royal Horticultural Society will no longer be represented on the Garden Bridge Trust after the retirement of Jim Gardiner.
Jim Gardiner was one of 13 trustees of the Garden Bridge Trust, alongside Joanna Lumley and Lord Davies.
The trust was set up to construct the controversial £185 million bridge between the South Bank and Victoria Embankment.
The Royal Horticultural Society told us in a statement: "Jim Gardiner has been executive vice president of the RHS and his role as a trustee of the Garden Bridge Trust has been to help and advise on the horticultural aspects of this project, primarily with the planting.
"On his retirement from the RHS on 28 October he will step down from this role.
"Should the Garden Bridge need any further help and advice on sustainable planting, the RHS will continue to provide horticultural advice, thereby supporting wildlife and the environment, but will not continue in a trustee role.
"Apart from Mr Gardiner's time, the RHS has not invested funds into the Garden Bridge and has never had a role in determining whether or not the project should go ahead. However, should the project proceed, as the UK's gardening charity, we would happily provide the GBT with our horticultural expertise and advice.
"The RHS is focusing on delivering our own £160 million investment in horticulture, including a new fifth garden, RHS Bridgewater in Salford, and increasing community projects across the UK."
The Garden Bridge Trust said: "Jim has been an excellent advisor and advocate for the bridge and we thank him for his hard work.
"The trust will look for a replacement to continue providing horticultural advice for this landmark project."