THE UK could be experiencing a once-in-a-decade wildlife phenomenon this year with a mass influx of painted lady butterflies, experts have said.

TV naturalist Chris Packham is urging people to take part in the world's largest insect citizen science survey, the annual Big Butterfly Count, to see if the painted ladies are arriving in their millions to the UK's shores this year.

The butterfly is a common immigrant from the Continent to the UK each summer where its caterpillars feed on thistles, but around once every 10 years there is a painted lady "summer" when millions arrive en masse.

Wildlife charity Butterfly Conservation, which runs the Big Butterfly Count, said unusually high numbers have been reported across Europe over the spring and early summer with large numbers now spotted crossing to the UK.

The last mass immigration took place in 2008 when around 11 million painted ladies migrated to the UK.

Mr Packham, who is vice president of Butterfly Conservation, said the painted lady migration was "one of the wonders of the natural world".

"Travelling up to 1km in the sky and at speeds of up to 30 miles per hour these seemingly fragile creatures migrate hundreds of miles to reach our shores each year.

"This butterfly undertakes an extraordinary 7,500-mile round trip from tropical Africa to the Arctic Circle every year - almost double the length of the famous migrations of the Monarch butterfly in North America.

"Signs across Europe are looking very promising, meaning that 2019 could be a very good year for the Painted Lady with high numbers already being recorded across parts of the UK.

"The butterfly can turn up anywhere so please take part in the Big Butterfly Count and look out for them - you could be witnessing a once in a decade butterfly phenomenon."

Participants are encouraged to spot and record 17 species of common butterfly, including painted ladies, and two day-flying moths in the UK during three weeks of high summer, to help experts see how the insects are faring.

Experts also say taking part in the count has benefits for those doing the count, with research showing that watching wildlife and spending time in nature can have positive benefits for mental health and well-being.

Mr Packham said: "The mental health benefits of spending time outdoors watching nature have been blindingly obvious to me for as long as I can remember.

"Immersing yourself in nature, even if it's just for a few short minutes, changes your perspective, it helps you slow down and notice what's going on around you and it opens a door to the overlooked beauty and drama of our natural world."

Butterfly Conservation is being supported by mental health charity Mind to champion the benefits of spending time in nature, and is sponsored by B&Q.

To take part in the count, which runs from July 19 to August 11, people just need to find a sunny spot anywhere in the UK and spend 15 minutes counting the butterflies they see, and then submit sightings online at www.bigbutterflycount.org or via the free Big Butterfly Count app.