Previous research in 2016 showed that 20% of plant species were under threat, but the "State of the World's Plants and Fungi 2020" report, published Wednesday, draws on the work of 210 scientists from 42 countries to reveal the scale of the problem, using improved data and methodology.
"Deforestation rates have soared as we have cleared land to feed ever-more people, global emissions are disrupting the climate system, new pathogens threaten our crops and our health, illegal trade has eradicated entire plant populations, and non-native species are outcompeting local floras," according to the report from RBG Kew, which carries out scientific research into plants and fungi as well as running a renowned botanical garden in west London.
And this is just what we already know about. Researchers say there are huge gaps in our knowledge of plants, and more work is needed to assess the conservation status of more species.
Alexandre Antonelli, Director of Science at RBG Kew, told CNN "it's just a race against time" as we are losing plants faster than we can name them.
"A world without those 40% is not the world we know today," Antonelli said.
While we don't know what the effects of losing them would be, he added, it could be "catastrophic," as we don't understand which species play important roles in particular ecosystems.
"Everything is linked up," Antonelli said.
Among those that we know are under threat are medicinal plants, with demand for naturally derived medicines threatening their survival. Some 5,411 medicinal plants were assessed in the study, and 723 are threatened.
Researchers say increased demand for herbal medicines is being driven a greater prevalence of certain chronic illnesses and the quest for new treatments.
While some medicinal plants are suffering overexploitation, Antonelli was keen to emphasize just how much plants and fungi have to offer.
The report recommends more funding for projects to find, name and conserve species that could provide solutions to some of humanity's biggest problems, such as food insecurity and climate change, before they go extinct.
For example, humans rely on just 15 plants to serve 90% of our food needs, adding to the problem of malnutrition and leaving us vulnerable to climate change, but scientists at Kew have identified 7,039 plant species that could be used as food.
"Harnessing this basket of untapped resources for making food and production systems more diverse and resilient to change, should be our moral duty to current and future generations," said Stefano Padulosi, former Senior Scientist at the Alliance of Biodiversity International and the International Center for Tropical Agriculture, who co-authored the food chapter of the report.
The situation is similar when it comes to producing energy. As things stand, we use six crops -- maize, sugarcane, soybean, palm oil, rapeseed and wheat -- to make 80% of global biofuel.
However, researchers have identified 2,500 plants that could be used to produce energy -- potentially a major boon for the 840 million people whom they estimate currently have no access to electricity.
"International collaboration can help us to identify the plants and fungi that will make clean, sustainable energy accessible to everyone," said contributor Mary Suzan Abbo, managing director of the Centre for Research in Energy and Energy Conservation at Makerere University, Uganda, who said the approach could be used to cut down on unsustainable use of wood and charcoal.
The report also describes a slew of species that have been identified recently, including two new relatives of cassava from Brazil, which experts say could future-proof the food crop by making it pest- or disease-resistant.
Developments like this are less likely as we lose more species, and Antonelli believes consumers need to make more sustainable choices while holding political leaders to account on policy.
"We really have to grasp the problem," he said, underlining that international cooperation will be key to reducing the threat and harnessing all that plants and fungi can do to help humanity.