The fifth national hen harrier survey showed the number of breeding pairs of the bird of prey in England fell from 12 in 2010 to four in 2016.
Scotland, which is home to 80 per cent of the UK population of hen harriers, saw its number of breeding pairs drop to 460 in 2016 from 505 six years earlier.
The number of breeding pairs in Wales fell to 35 from 57 while Northern Ireland also experienced a drop from 59 to 46.
Hen harriers are the most threatened birds of prey in the UK, with conservationists blaming illegal persecution as well as the historic destruction of vital heather moorland and forestry habitats.
In North Yorkshire, five tagged hen harriers were reported missing between November 2016 and February this year.
The RSPB believes there should have numerous hen harriers in Yorkshire but none have been known to breed in the region for several years.
Martin Harper, the RSPB’s conservation director, warned: “The latest figures back up a continued trend that we have seen for more than a decade - hen harrier numbers are on the decline throughout the UK.
“The illegal killing of this bird of prey is a significant factor behind the diminishing numbers and a large barrier stopping their recovery.
“Without purposeful action from all, including governments across the UK and the shooting industry, we may see hen harriers once again lost from more parts of the country.”
The charity last year withdrew its support for the government’s Hen Harrier Action Plan over its perceived failure to deliver urgent action and change in behaviour which the RSPB says is needed to save Britain’s rarest bird of prey from extinction. Instead it has called for a USA-style licensing system for grouse moors.
The Moorland Association continues to back the government’s plan.
Simon Wotton, lead author of the latest study, said: “We hope these results will convince all those who are in a position to help hen harriers to take positive steps to ensure their protection and rebuild the UK’s hen harrier population for people to enjoy for generations to come.”