Some 1,753 are non-native flora that compete with native species and can become invasive. Examples include the New Zealand Pigmyweed and Sitka Spruce.
Comparisons to previous surveys indicate that the ranges of 53% of all native plants in Britain have declined, whereas 58% of non native species have expanded their ranges.
READ MORE: 9 things we’ve learned from the updated invasive species strategy
Native species have also fared badly in Ireland, with 56% declining. In comparison, non native species have "thrived with 80% of them increasing."
The report states: “There are now more introduced plants growing in the wild in Britain than natives, with many originating from gardens and then spreading to establish self- sustaining populations.”
Released by the Botanical Society of Britain and Ireland (BSBI) yesterday, the report outlines the combined impact of climate change, modern farming practices, and loss of habitats on plant species.
Written by researchers at BSBI and the UK Centre for Ecology and Hydrology, it reveals that just under half (49%) of all plant species are estimated to have declined in range in Britain since the 1950s, whereas only a fifth (20%) have increased.
“While a warmer climate is likely to hasten the arrival of more Mediterranean species such as tongue-orchids, sadly it may seal the fate of arctic species such as Snow Pearlwort Sagina nivalis,” according to the report.
The findings are "catastrophic" for native species, according to Dr Kevin Walker, head of science at the BSBI and a co-author of the Plant Atlas 2020.
"The loss of grasslands, heathlands and other habitats would be really shocking for someone brought up in the 1950s," he said.
"There are lots we can do to reverse these declines, but the most important are to increase the protection plants receive, extend the habitat available to them, and place their needs at the very heart of nature conservation.”
Climate change is creating winners and losers in flora. Plants such as alpine lady-fern and alpine pearlwort are suffering shrinking ranges. But others, including the bee orchid, early meadow-grass and mossy stonecrop, have expanded further afield.
The report calls for legal protection for plants and wildlife to be strengthened, an increase in “high-quality habitat” as well as sustainable land management.
“There has never been a greater need for a robust plant monitoring and surveillance programme that tracks trends in relation to habitats and key drivers,” it states.
And "plant blindness" needs to be tackled by building an appreciation and understanding of the importance of flora.
Commenting on the findings of the report, Craig Bennett, chief executive of The Wildlife Trusts, said: “The government’s new farm environment schemes must do what was originally promised and reverse the decline of nature in our agricultural landscape.”
He also called for greater protection for Local Wildlife Sites and for the government to honour its pledge to halve nutrient pollution by 2030.
A DEFRA spokesperson said: "Protecting our native species, biodiversity and plant health is fundamental to halting the decline in nature, and we have demonstrated unwavering commitment in these areas through the Environmental Improvement Plan and its ambitious targets."
They added: "We’re already taking action and this year have introduced the Plant Biosecurity Strategy and Invasive Non-Native Species Strategy to reduce the impacts of pests and disease and ensure that our native species are able to flourish for generations to come.”