Sir David Attenborough, who is launching the Study of Nature report, called it a "stark warning" but also "a sign of hope".
Sir David said: "For 60 years I have travelled the world exploring the wonders of nature and sharing that wonder with the public. But as a boy my first inspiration came from discovering the UK's own wildlife.
"Our islands have a rich diversity of habitats which support some truly amazing plants and animals.
"We should all be proud of the beauty we find on our own doorstep; from bluebells carpeting woodland floors and delicately patterned fritillary butterflies, to the graceful basking shark and the majestic golden eagle soaring over the Scottish mountains.
Dr Mark Eaton, a lead author on the State of Nature report, said it reveals wildlife in the UK is "in trouble" and that overall "we are losing wildlife at an alarming rate".
More than 750 species assessed on international "Red List" criteria, which measures the threat of extinction, are at risk of vanishing from the UK altogether, the experts warned.
The abundant wildlife of the UK's Overseas Territories is also under threat, with 90 species at a high risk of global extinction.
More than three-quarters of 155 species of birds, animals, butterflies and moths which are considered a priority for conservation action - including some of the country's rarest and most vulnerable wildlife - have seen declines in the past 40 years.
Species which rely on particular habitats have fared worse, and the declines in wildlife come against a background of massive loss of natural habitats in the past two centuries, including lowland heathland, lowland meadows in England and Wales and Scotland's blanket bog.
Wildlife in the UK is "in trouble", with almost 2,000 species of birds, animals, insects and plants known to have declined in the past half century, experts warned.
Some 60% of the 3,148 British species studied have seen a reduction in numbers or range, with 31% suffering major declines, according to a report released by a coalition of conservation and research organisations.
Well-loved species from skylarks to hedgehogs are struggling in the face of threats that include loss of habitat, humans' management of the countryside and the increasing issue of climate change.
Almost three-quarters of butterfly species have declined in the past decade, plants are going extinct from some counties at a rate of one every other year and the UK has lost around 44 million breeding birds since the late 1960s, the State of Nature study found.