Swifts and house martins have been added to the UK's critical red list of birds which are significantly under threat.

For the first time the pair have been moved from amber to red on the warning light system because alarming drops in their numbers has triggered the highest level of 'conservation concern'.

The greenfinch and Montagu's harrier have also both been added to the 2021 update - taking the number of birds wildlife experts are most concerned about to 70 - while the white-tailed eagle has been downgraded thanks to dedicated conversation efforts in the last few years to boost its numbers.

Published today in the British Birds journal, the latest assessment is compiled by a coalition of leading UK bird conservation and monitoring organisations, which regularly updates the status of 245 species of bird found in the UK, Channel Islands and Isle of Man.

But the latest data reveals that more than one quarter of these - or around 29% - are in serious trouble.

Swifts and house martins, which often rely on man-made structures for nesting sites, are joining other iconic species already on the critical list including the cuckoo, turtle dove and nightingale.

Most of the species placed on the red list, which has grown by three since its last update in 2015 as a result of the white-tailed eagle's downgrading, are on there because of their severe decline, having halved in numbers or range in the UK in recent decades.

But with many of the 70 species included migratory, the report can also help conservationists to identify issues that require work internationally to halt global declines of some of the planet's most threatened bird species.

The greenfinch has this year moved directly from the green to the red list after a population crash associated with trichomonosis, a parasite-induced disease which prevents the birds from feeding efficiently. Many other garden favourites also feature on the UK's red list including the starling, house sparrow and now swift.

The RSPB’s CEO, Beccy Speight said more work needs to be done to protect certain species.

She said: "This is more evidence that the UK’s wildlife is in freefall and not enough is being done to reverse declines. With almost double the number of birds on the red list since the first review in 1996, we are seeing once common species such as swift and greenfinch now becoming rare.

"As with our climate this really is the last chance saloon to halt and reverse the destruction of nature. We often know what action we need to take to change the situation, but we need to do much more, rapidly and at scale. The coming decade is crucial to turning things around.”

Dr David Noble, principal ecologist for monitoring at the British Trust for Ornithology and the report's co-author, said the decline in swifts was being caused by a lack of food and suitable nesting areas.

He explained: "Familiar to all by their impressive aerial and vocal displays, swifts have been in steady decline in England since the start of the Breeding Bird Survey in the mid-1990s and numbers are now estimated to be less than half (down by 58%) of what they were in 1995.

“Causes of the decline have been attributed to reductions in the aerial insects that this species depends on, as well as disappearance of suitable nesting sites when urban and industrial buildings are refurbished. Tackling the first issue is challenging because it is related to land use, climate change and pesticides but there are measures that can be taken now to protect and maintain access to nest sites, minimise disturbance and provide alternatives."

Natural England said the latest red list report should act as a 'wake up call'.

Chairman of the organisation Tony Juniper said: “This report provides yet another wake-up call as to the plight of our country’s declining wildlife, revealing how some of our much loved and once common birds, including swifts and house martin, are now of critical conservation concern.

“Natural England is working with many partners to help pull more than 200 species back from the brink and we have seen real success with some of our most iconic birdlife recovering, such as bittern and cirl bunting. We must, however, do a great deal more if we are to achieve our national ambitions for nature recovery.

“Central to our plans must be a major programme of nature recovery, to begin in earnest the restoration of large areas of connected natural habitats, which in turn will enable the populations of many species to increase. Central to this will be the creation of the wildlife-rich Nature Recovery Network that lies at the heat of Natural England’s work.”