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Blink and you could miss them - but mysterious sightings of an extremely rare butterfly have set the hearts of enthusiasts fluttering.

The species, previously described as extinct in Britain for nearly 100 years, has suddenly appeared in countryside on the edge of London.

Small numbers of black-veined whites have been spotted flying in fields and hedgerows in south-east London.

To the non-expert, they could easily be mistaken for the common or garden cabbage white butterflies seen in Britain every summer.

But there's nothing common about the black-veined white on this side of the Channel.

First listed as a British species during the reign of King Charles II, they officially became extinct in Britain in 1925.

This month they have mysteriously appeared among their favourite habitat: hawthorn and blackthorn trees on the edge of London, where I and other naturalists watched them flitting between hedgerows.

As their name suggests, they are a medium-sized white butterfly with distinctive black vein markings on their wings.

The charity Butterfly Conservation, which monitors butterfly numbers in Britain, told the ´óÏó´«Ã½ the insects will have been released, but they don't know by who or why.

They added that while it's lovely for people to be able to see them, it probably does not signify a spontaneous recovery of an extinct species.

But some experts disagree with the charity about the cause of the species' reappearance. They argue a black-veined white could have arrived in the UK from continental Europe, where they're common, and laid her eggs amongst the trees near southeast London.

Update 1 September: A line has been added to this article to reflect that some experts believe the butterfly species could have reappeared due to natural causes.