The RHS says 350,000 fewer people have visited annually since the works began in 2022.
This is a loss of £6 million for the horticulture charity.
Mr Titchmarsh is Vice President of the RHS.
He’s calling for government intervention and says the losses are “catastrophic not only for the RHS, but for the whole of the UK”.
Click here to view the petition.
We’ve contacted Highways England for comment.