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A swarm of painted lady butterflies land on a Carolina cherry tree as they migrate north from Mexico through Encinitas, California, U.S. Image Credit: Reuters

London: Millions of butterflies are expected to start arriving in Britain this summer, in a once in a decade phenomenon.

Wildlife lovers are being urged to look out for the Painted Lady butterfly which has begun arriving from the continent as part of its 7,500-mile annual migration round-trip, according to the wildlife charity Butterfly Conservation.

"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," said wildlife TV presenter Chris Packham, who is vice president of the charity.

The last mass influx took place in 2009 when around 11 million Painted Ladies arrived in the UK, where their caterpillars feed on thistles.

Travelling high up at speeds of up to 30 miles per hour, the butterfly migrates from Africa to the Arctic Circle in an annual journey that can take up to six generations.

Those Painted Ladies returning to Africa in the autumn are several generations removed from their ancestors who left Africa earlier in the year.

Packham is calling on nature-lovers to take part in the Big Butterfly Count which begins on Friday to see whether a 'Painted Lady summer' is actually taking place.

Butterfly Conservation has emphasised the mental health benefits of counting butterflies and the mental health charity MIND is supporting the count.

"Being outdoors in green environments can help us deal with negative feelings and experiences like depression," said Rachel Boyd from MIND.

The Big Butterfly Count runs to Aug. 11.