A garden which is a “centre of excellence” for horticulture and attracts more than a million visitors a year is under threat from a road-widening scheme, the Royal Horticultural Society has warned.

Plans to widen the A3 in Surrey as part of improvements to junction 10 of the M25 include options that would be a huge “garden grab” on Grade II* listed RHS Wisley Gardens, the society said.

The garden could lose 10,000 square metres (2.5 acres) of woodland and 500 trees – including one planted by the Queen to mark her silver jubilee – if one of the options being considered by Highways England goes ahead, it warned.

The Queen at RHS Wisley GardensThe Queen at RHS Wisley Gardens (Fiona Hanson/PA)

RHS ambassador Alan Titchmarsh has called on the UK’s army of gardeners to oppose the plans, saying: “We must stand together and protect our gardens.”

The RHS said there were two options to widen the A3, one on the road’s east side, and one on the west alongside the century-old garden.

The west option would take out the protective bank of trees which separates the busy road from the trials field where varieties of blooms are assessed, as well as part of the woodland garden, worsening noise and air pollution at the garden.

Historic trees that are more than 100 years old, five trees identified as “threatened and endangered in cultivation”, and huge giant redwoods would be lost if the scheme went ahead, the charity said.

Titchmarsh said: “This potential garden-grabbing plan would be another unacceptable example of this Government’s poor perception of horticulture and lack of appreciation of the vital role that plants play for the environment, for the nation’s health and well-being and for the UK economy.

“Wisley is the UK’s centre of excellence for horticulture and horticultural science and helps millions of people to garden and grow plants.

“I’m calling on the UK’s army of 27 million gardeners to make it known that a disregard for these important trees and lack of appreciation of the national importance of this garden would not be acceptable if the short-sighted and environmentally damaging option was chosen.”

The RHS is urging Government agency Highways England to choose the option on the east side, which it said would not take any woodland or fell trees at Wisley.

Highways England project manager Hugh Coakley said: “Highways England cares about the environment, and protecting the special habitats around Wisley is a priority for us as we develop our proposals for major improvements at the junction of the M25 and A3 in Surrey.

“It is one of the busiest motorway junctions in the country, and upgrading it will improve people’s journeys and make both roads safer.

“Both the options we consulted on in the winter include better, safer access to Wisley Gardens and we will continue to work closely with RHS Wisley as we take the project forward. We look forward to announcing the results of the consultation soon.”