The adder, Britain’s only venomous snake, has for the first time been confirmed as being active in every month of the year.

Adders normally hibernate underground from October to March, a strategy designed to enable them to survive a cold winter, but with warmer weather have now been seen throughout the year.

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They are declining as a result of increasing habitat loss, persecution, predation, disturbance and inbreeding. A shorter hibernation period and becoming more active in winter is likely to accelerate their demise, as snakes will be much more susceptible to any cold weather that follows a warm spell.

Following the record warm temperatures in February, adders have been seen in every calendar month. While the adder is well adapted to cold weather, occurring further north in the world than any other snake species, this is new behaviour and experts warn it is a clear sign of the impact climate change is having on wildlife.

Nigel Hand, a trustee of Amphibian and Reptile Groups of the UK and one of the country’s leading adder experts, confirmed the trend.

“I’ve seen adders in every month of the year except December, but have had reliable reports of them being active then in Wales,” he said. “Adders have already been recorded as being seen throughout the year in a national park in the Netherlands, so I have no doubt they are now active throughout the year in Britain as well.”

The recent unprecedented temperatures have resulted in adders emerging from hibernation up to a month early. Being active in winter brings them into greater contact with humans and dogs, leading to increased disturbance at their hibernation sites at a time when they are particularly vulnerable because they are sluggish and bask in the open.

Facebook Twitter Pinterest The adder used to be widespread across Britain but has declined markedly in recent years, particularly in middle England. Photograph: Nicholas Milton

Adders are protected by law and are shy reptiles that will always try to escape unless cornered or molested. Although they are venomous, no one has died from an adder bite in Britain in more than 40 years and conservationists have in recent years tried to tackle the reptiles’ “image problem”.

However, dogs off leads will often attack adders and there have already been reports this year of adders biting dogs in the New Forest. As a result the TV presenter Chris Packham, who lives in the area, has called for dogs to be banned from nature reserves in the People’s Manifesto for Wildlife he launched last year.

The adder used to be widespread across Britain but has declined markedly in recent years, particularly in middle England. Research published in the Herpetological Journal has warned that at current rates of decline, the adder could be restricted to just a few sites in the UK within 10-20 years.