Image copyright PA Image caption The large blue butterfly is thriving on reserves in Gloucestershire and Somerset

A rare butterfly once declared extinct in the UK is now being seen in record numbers after being reintroduced.

Conservationists said there was a population of more than 10,000 large blue butterflies on reserves in Gloucestershire and Somerset.

The last large blue butterfly colony was on Dartmoor in Devon and it was declared extinct in 1979.

The population in the UK is now believed to be largest concentration of the species anywhere in the world.

The butterfly, with a wingspan of more than two inches, was brought back from the dead with the reintroduction of stock from Sweden in 1984 and has thrived since then.

The key to the success of the programme was an understanding of the butterfly's bizarre life-cycle which is similar to that of the cuckoo.

The caterpillar tricks red ants into believing it is one of their own grubs and is carried underground.

It feeds on the larvae around it until it emerges 10 months later to live for just a few weeks as a butterfly.

Conservationists said the story of the large blue showed that the decline of globally threatened species could be reversed.