One in five of the world's plant species – the basis of all life on earth – are at risk of extinction, according to a landmark study published today.

An orchid is seen at the University of Costa Rica in Cartago, close to San Jose. The greatest peril to plants came from human-induced habitat loss, mostly the conversion of natural habitats for crops or livestock, research by respected Kew Gardens in London has revealed. (AFP/File/Yuri Cortez) At first glance, the 20% figure looks far better than the previous official estimate of almost three-quarters, but the announcement is being greeted with deep concern.

The previous estimate that 70% of plants were either critically endangered, endangered or vulnerable was based on what scientists universally acknowledged were studies heavily biased towards species already thought to be under threat.

Today the first ever comprehensive assessment of plants, from giant tropical rainforests to the rarest of delicate orchids, concludes the real figure is at least 22%. It could well be higher because hundreds of species being discovered by scientists each year are likely to be in the "at risk" category.

"We think this is a conservative estimate," said Eimear Nic Lughadha, one of the scientists at Kew Gardens in west London responsible for the project.

The plant study is also considered critical to understanding the level of threat to all the natural world's biodiversity, said Craig Hilton-Taylor of the International Union for the Conservation of Nature (IUCN), which runs the world's offical "red list" of threatened species. "Plants are the basis of life, and unless we know what's happening to plants it has many implications," said Hilton-Taylor.

The results will be presented to world leaders meeting at Nagoya in Japan in October to discuss the world's biodiversity crisis, along with new red lists for vertebrates and several groups of the planet's millions of invertebrate species.

"This is a base point," said Lughadha. "What we do from now is going to lead to the future of plants. We need to challenge the idea that plants are there to be exploited by us, we need to move to a system where we're nurturing plants much more carefully [and] actively taking steps to conserve them."

Politicians and conservation experts will also be told that by far the biggest threat to plants is human – rather than natural – causes, especially intensive agriculture, livestock grazing, logging and infrastructure development.

Caroline Spelman, the environment secretary, who will travel to Japan for the final talks, said the results were deeply troubling. She added: "Plant life is vital to our very existence, providing us with food, water, medicines, and the ability to mitigate and adapt to climate change."

Scientists randomly selected 7,000 species from across the major plant groups as a representative sample of the estimated 380,000-400,000 so far known to science. Of these, 3,000 were found to have too little information to begin making an proper assessment – a result that was expected and so built into the selection process.

The remaining 4,000 species were assessed and the level or risk based on a combination of the absolute number of plants estimated in the wild, the known decline, and the total area in which they are thought to live.

Of the 4,000, 63% were found to be of "least concern", 10% near threatened, 11% vulnerable, 7% endangered and 4% critically endangered. Another 5% were rated "data deficient".

The proportion of plant species deemed at-risk is similar to that of the IUCN's red list for mammals, worse than that for birds (less than 10% at-risk) and better than the number for amphibians (more than a quarter under threat).

Nearly two-thirds of threatened plant species are found in tropical rainforests, five times the proportion for the nearest other habitats – rocky areas, temperate forests and tropical dry forests. This is because of their huge density of biodiversity and the widespread risks of logging and clearance for other agriculture, said analysts.

Previously the red list for plants contained assessments for a greater number of plants – about 12,873 or 3% of known species – but was not considered representative because scientists had focused on at-risk species so that they could get attention and funding for conservation.

The assessment was done using experts and collections at the herbaria at Kew Gardens, the Natural History Museum in London and Missouri Botanical Garden in the US, plus specialist experts from the IUCN.

From pines to snowdrops to rosewood – six of the endangered plants

Wollemi pine (Wollemia nobilis) – critically endangered

The wollemi pine was discovered in 1994 in Wollemi National Park, Australia, and there fewer than 50 mature individuals are known. Its long-term regeneration from seed is unknown but seems doubtful due to competition with other trees. Its small size and limited range means it is at risk from any chance event such as fire or the spread of disease.

Common snowdrop (Galanthus nivalis) – near threatened

The common snowdrop was once widely distributed in the east Carpathian mountains in central and eastern Europe. Although it is widely naturalised, including in the UK, during the past decade its native distribution has been considerably reduced, due mainly to habitat loss through the increase in residential developments and recreational land use.

Rosewood (Dalbergia andapensis) – critically endangered

D. andapensis is a species of rosewood, a highly valued timber sed in the production of fine furniture and musical instruments. It is estimated that 52,000 tonnes of rosewood and ebony were logged in north-east Madagascar in 2009, and this habitat is itself under threat from conversion to agriculture for a growing rural population.

Wood bitter-vetch (Vicia orobus) – least concern

Wood bitter-vetch is a rare species found through much of western Europe, including the British Isles, at woodland margins, field edges and rocky places, often on limestone. In Ireland it is considered to be threatened as a result of habitat loss, and is being protected by the National Botanic Gardens of Ireland.

Whited's milkvetch (Astragalus sinuatus) - Critically Endangered

Whited's milkvetch is restricted to a tiny area of the state of Washington, USA. Its dry hillside habitat is threatened by invasive, non-native species, by grazing and by agriculture. Seeds have been collected and banked by the Berry Botanic Garden Seed Bank for Rare and Endangered Plants of the Pacific Northwest and the Miller Seed Vault, University of Washington Botanic Gardens.

Encephalartos altensteinii – vulnerable

E. altensteinii is found in coastal regions of the eastern cape, South Africa, where the number of individuals has declined by more than 30% in the past 50 years. Large numbers have been removed from its native habitat, including 438 plants in one poaching incident in 1995, mainly by horticultural collectors for pot plants or medicinal use.