Almost 100,000 people signed a petition to save jobs at Kew Gardens put at risk by Government cuts.
It comes after Kew Gardens announced it faced a £5m shortfall that would see 125 jobs lost across Kew and Wakehurst Place, Sussex, in the coming year.
The petition was handed to David Cameron at 10 Downing Street this week by union representatives, Prospect union members at Kew and MPs Zac Goldsmith and John McDonnell.
Richmond Park MP Zac Goldsmith said: "It’s not clear to me that Kew Gardens can sustain its extraordinary work with the level of cuts being imposed.
"Kew Gardens is a national jewel, and it would be a tragedy if its work and reputation were to suffer as a result of excessive cuts."
Fears are growing for the work at Kew Gardens, which has already lost about 50 posts, due to plans to cut a further 50 to 70 posts across the two centres.
GMB regional officer Paul Grafton said: "The Government needs to listen to the experts and restore Kew’s funding to levels that will ensure the world’s plant life remains protected now and for the future.
"Never before has Kew faced such a significant threat to its future."
The job cuts are in response to the loss in Government funding to Kew, which has fallen from 90 per cent in 1983 to below 40 per cent this year.
Speaking to the Richmond and Twickenham Times earlier this year Sir David Attenborough said Kew Gardens played an "absolutely crucial role in looking after our botanical heritage and our botanical future".
He added: "The Government and the scientific departments should recognise that and support it properly."
A spokesman for Royal Botanic Gardens, Kew said: "We are grateful for the incredible public affection shown for Kew from across the globe.
"We want to reassure our supporters, donors and partners that we have a plan in place to combat the deficit, and to put Kew on a firm footing for the future so that we can continue to make a vital contribution to plant science, conservation, education and horticulture."