Britain's wildlife is being pushed to "the brink of a crisis" as sub-zero temperatures continue to grip the nation, according to conservationists.
The Royal Society for the Protection of Birds (RSPB) is organising emergency feeding of several threatened species, including bitterns and cirl buntings.
The RSPB is also asking people to feed garden birds, which are struggling to find food in the freezing weather.
The harsh winter could hit bird numbers "for many years to come", they warn.
The UK's Met Office recorded the winter's coldest day on Friday, when temperatures plunged to -22.3C (-8.1F) in Scotland's Highlands.
With the icy weather predicted to last at least another week, this winter could be the single greatest wildlife killer of the new millennium
Severe weather warnings of heavy snow are in place for eastern England, and forecasters warn that the coming days could be even colder.
"The extremely hard winter spanning 1962 and 1963 was arguably the single event that had the greatest impact on wildlife within living memory," said Mark Avery, the RSPB's conservation director.
"With the icy weather predicted to last at least another week, this winter could be the single greatest wildlife killer of the new millennium."
Dr Avery said that recent conservation measures - combined with mild winters - had seen a welcomed increase in the number of some threatened species, such as the bittern.
But he added: "Now that the barometer has swung rapidly in the other direction, some species face a crisis, forcing us to take immediate action."
RSPB volunteers have begun "emergency feeding initiatives", which include putting out sprats at key sites for bitterns.
The birds are finding it difficult to catch fish in ice-locked wetland areas.
The society says that it is being inundated with calls from the public, who are reporting unusual visitors to bird feeders in gardens - including woodcock, snipe and grey wagtails.
The freezing weather means that gardens have become havens for wildlife, it adds, as birds and animals are forced to venture further afield in the search of food.
The RSPB has joined forces with other conservation groups - including Natural England, the British Trust for Ornithology (BTO) and the Wildfowl and Wetlands Trust - to urge people not to disturb flocks of wetlands birds.
They say ducks, swans, geese and other wading birds cannot afford to waste energy that they cannot readily replace.