Kew Scientists concluded that 22 per cent - more than 80,000 - plant species are in danger of dying out from habitat loss.
Plants most at risk of being wiped out include the Amazon lily, Wollemi pine, dwarf palm in Madagascar and Whited's milkvetch.
Kew holds more than one in eight of all known plant species and its researchers worked with colleagues from London's Natural History Museum and the International Union for the Conservation of Nature.
Scientists will continue to monitor the "Red List" of endangered species and use it to create plant seed banks and target regions for conservation work.
Kew's Dr Eimear Nic Lughadha, who led the study, said: "There was no good data about how much plant biodiversity is being lost until now. With mammals and birds, it was possible to assess everything because there are thousands rather than hundreds of thousands of species.
"But for plants there was no effective assessment of the status of each species, so we were approached to take a worldwide sample. It is important we now have a baseline so that in five years' time we are able to measure any change. I hope we can drive that 22 per cent down." The report comes ahead of next month's UN Biodiversity Summit in Nagoya, Japan.
Quangos under threat
Kew was among 100 quangos under threat in a list drawn up by the Cabinet Office ahead of the Government's spending review. But Kew's Dr Eimear Nic Lughadha said the Red List was an example of Kew work that matches the Government's goals.
"We have not got a shortage of reasons to exist," he added. "Our mission is very clearly aligned to the Government's green agenda."
- To be abolished Agricultural Wages Board, British Waterways, Darwin Advisory Committee, Pesticides Residue Committee
- Responsibility devolved to relevant local authority London Thames Gateway Development Corporation, Olympic Park Legacy Company
- To be merged English Heritage
- Under review CABE, Environment Agency, Kew, Natural England, National Forest Company, Historic Royal Palaces, AHDB, Forestry Commission