This year's spring arrived in November, with insects active and flowers blooming months ahead of schedule, nature sightings from the public show.
The Woodland Trust's Nature's Calendar scheme, which asks the public to track signs of the seasons, has received more than 64 records of early spring activity starting in November.
But the early spring, prompted by mild weather in the last two months, could be brought to a halt as a cold snap sweeps in.
Mild conditions temporarily disturbed insects from hibernation, with a small tortoiseshell butterfly spotted flying outdoors on Christmas Day in Merthyr Tydfil, Wales, and a red-tailed bumblebee on Boxing Day in Somerset.
A red admiral was seen on December 17 in Cambridgeshire, five months ahead of the May 7 average date the butterfly emerges, the trust said.
Other sightings include 24 records of snowdrops flowering around the country more than a month earlier than normal, the earliest of which was in Southampton on November 30.
Though hazel trees usually flower in early March, there have been 23 hazel flowering records, with the first on December 1.
An oxeye daisy, which normally blooms from mid-April to early June, was seen flowering in Gloucestershire on December 28.
Even birds are ahead of the times: song thrushes have been heard in 11 places since December 5, and are increasingly being reported singing all winter, even though they are expected to start their chorus in March.
Blue tits were also spotted checking out a nesting box on December 26, several months ahead of their usual nesting times.
The UK has seen a relatively mild winter so far, with average temperatures 1.1C above the long-term average in November and 1.9C higher in December, according to the Met Office.
But spring may be halted in its tracks and delicate species could suffer as the forecasters are warning of a dip in temperatures this week, with frost, ice and even snow, and cold weather could stick around for some time.
Kate Lewthwaite, citizen science manager for the Woodland Trust, said: "Once again - despite being in the throes of January - flora and fauna are reacting to milder climates, and spring seems to have sprung early.
"We were far from a white Christmas, with hazel flowers and snowdrops being spotted by our citizen scientists across the country.
"Data like this has continuously brought into question the way we think about the seasons, and to see spring in December no longer seems unusual."
Nature's Calendar data shows buds are bursting into leaf earlier and leaves falling later, backing up Met Office research suggesting the plant-growing season has extended by a month.
Dr Lewthwaite added: "The more data we have, the better we will understand the effects of warm winters, cold snaps and heatwaves. In short, we need more Nature's Calendar recorders."