With frosty mornings and snow sweeping across the country, it may feel like there is no sign of winter ending - but nature experts say spring is already on the way.

Sightings of snowdrops, hazel catkins and even ladybirds and butterflies have been submitted to the Woodland Trust's Nature's Calendar scheme, which asks the public to report natural events which mark the changing seasons.

So far in 2015, there have been reports of snowdrops at 120 locations across the country, hazel catkins at 74 spots and even sightings of ladybirds, small tortoiseshell, peacock and red admiral butterflies, the Woodland Trust said.

A warming climate has seen spring coming earlier, and three of the last four years have seen some of the earliest spring averages the Trust has compiled from its monitoring of the seasons in Nature's Calendar for the last 15 years.

Research looking at nature records stretching back to the 18th century has found flowers over the last 25 years blooming up to 12 days earlier than previously.

This year's sightings come after the warmest year on record for the UK.

The current freezing conditions may halt the march of spring, but conservationists say that causes less of a problem than a late cold snap, as happened in 2013, when more species are likely to be awake from hibernation or already flowering.

Dr Kate Lewthwaite, Woodland Trust citizen science manager, said: "People still seem to be surprised to see snowdrops and butterflies in early January, but our warmer climate in recent years means this is now pretty common.

"Records added to Nature's Calendar by the public have enabled scientists to learn how trees, plants and wildlife adapt to our changing climate.

"If we can ensure our natural environment is diverse, resilient and interconnected it will make it easier for wildlife to adjust too."

The Trust said it would be working in partnership with the British Science Association to estimate the rate spring progresses this year.

National Trust's national specialist on nature and wildlife, Matthew Oates, said: "Spring has been calling for some time, due to a mild autumn, December and early January. The signs are all around us.

"Spring's advance has now been checked by the arrival of the first cold snap since March 2013. Perhaps spring needed slowing down, as early springs are risky and almost always end in tears."

But he urged the public to look out for the signs of spring and report them to the Woodland Trust's Nature's Calendar.