Top television naturalist Sir David Attenborough has been in Somerset celebrating the 25th anniversary of the reintroduction of the large blue butterfly into Britain.

Somerset now has the location of the largest concentration of large blues in the world.

Sir David was among the speakers at the celebration event, attended by many leaders of the UK's scientific and conservation communities.

He said: "The restoration of the large blue butterfly to Britain is a remarkable success story, illustrating the power of ecological research to reverse damaging environmental changes.

"It is, moreover, a tribute to the dedication of many practical conservationists who have skilfully recreated its specialised habitat in our countryside."

Professor Lord May of Oxford, recent president of the Royal Society and former chief scientific adviser to the Government, said: "The recovery of the large blue butterfly is the world's largest-scale, longest-running successful conservation project involving an insect. It illustrates perfectly how the application of sound science can be used to solve some of the apparently intractable problems that face conservationists worldwide today.

The large blue is one of the world's most threatened species. The British reintroduction was based on major scientific breakthroughs, and the development of a pioneering model for conservation which is now being used to restore rare butterfly species across Europe.

Experts had always hoped that the Green Down reserve near Street could support 150,000 eggs that could produce 5,000 butterflies, which is a higher number than anywhere else in the world, and it has exceeded that.

The Somerset Wildlife Trust closes the site for six weeks while the large blue is on the wing. Enthusiasts can see the butterfly at an annual open day where permits are required.

The National Trust's Collard Hill site, near Street, also hosts a large colony of the butterfly and is always open to the public.

The flight season is starting now and National Trust staff and volunteers will be on the site every day over the next few weeks to share advice on how to track the butterflies and photograph them.