The charity Butterfly Conservation says that the rare moth typically found in the Mediterranean has become established in the UK.
The flame brocade moth appears to have started a new colony on the south coast of England.
Researchers from the charity have attributed the arrival of an unusually high number of migrant species to the recent warm weather.
They described this year's migration season as the best in years.
Some of the insects will have flown for three or four days to get to the UK, on the back of a helpful southerly tailwind.
The organisation's head of moth conservation, Mark Parsons, said: "Autumn is usually a good time for immigrant species, but it's the sheer number and diversity this year that's special."
The flame brocade, perhaps the most significant arrival, was first spotted by chance in a back garden in Sussex.
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By Richard Westcott BBC News
