Nearly half of bird species are threatened while almost a third of amphibians, reptiles, fungi and lichen and a quarter of land-living mammals are at risk of vanishing, the newest State of the Nature report found.
More than half of flowering plant species no longer grow where they used to, with climate change and intensive farming being the biggest reasons why the UK has seen an average decline of all living species of 19% since monitoring began in 1970.
Even before then, UK wildlife had been depleted by centuries of habitat destruction, unsustainable farming practices and persecution.
It means more than half of the UK’s plant, fungal and animal life has been killed off.
Professor Richard Gregory, the RSPB’s head of monitoring conservation science, said: “The sobering message is that the state of UK nature and the wider environment, based upon the indices that we’ve got, continues to decline and degrade.
“At the same time, we’ve never actually had such a good understanding of the state of nature in the United Kingdom and we’ve never had such a good understanding of how we might fix it."
Led by the RSPB and backed by over 60 research and conservation organisations, the State Of Nature report relied on thousands of volunteers recording sightings of various species across the UK, its crown dependencies and overseas territories.
They assessed more than 10,000 species as well as the state of the habitats vital for their survival, which require "very strong action to restore them to where they need to be", Professor Fiona Mathews of the University of Sussex said.
Only 7% of broadleaved woodland was found to be in good condition, with an increase of 7,000 hectares needed every year, while only 25% of peatland, also an important natural carbon sink, and 50% of salt marshes remain in good condition.
Professor Mathews said small mammals such as harvest mice and field voles are "disappearing before our eyes" because the habitat on which their survival depends is being destroyed, with knock-on effects for owls and other predators.
Lichens, which are a mix of fungi and algae and thrive in clean, wet, undisturbed forests, growing in a psychedelic mix of patterns and colours on tree branches, have recovered somewhat since the 1970s thanks to reductions in sulphur dioxide from industry.
Nearly half of lichen species are still declining however because of ammonia, which mostly comes from muck-spreading, slurry and fertilisers used on farms.
Insects that pollinate and those that prey on pests such as ladybirds, ground beetles and wasps are also disappearing in certain areas - falling by 18% and 34% respectively.
Dr Francesca Mancini, of the UK Centre for Ecology and Hydrology, warned that these losses could damage farmers financially.
She said: “We can think that further losses in pest control species for example could lead to economic losses for farmers and also a greater reliance on chemical pesticides, which then in turn is going to have more consequences for biodiversity."
Some species, such as dragonflies, have improved thanks to rivers being cleaner than they were in the 1970s, while conservation projects from the Cairngorms to Cambridgeshire and Lyme Bay, show that restoration can and does help plants and animals to return.