A vast bunker built to safeguard the future of endangered plant species was officially opened by the Prince of Wales today.

Millions of seeds are being banked in the vault, buried deep in the countryside, in the biggest conservation project of its kind in the world.

Prince Charles was shown around the £80 million Millennium Seed Bank at Ardingly, near Haywards Heath, to see how scientists aim to save plants across the globe from the threat of extinction.

But Charles, in joking mood, took time in his speech to leave aside the serious business at hand, choosing to refer back to the time when he was ridiculed for talking to plants.

To loud laughter he told the audience: "I have a particular interest in this. I want to make certain that I have some plants left to talk to, as I am sure you would agree because I am sure there are some botanical conversationalists among you."

The Seed Bank - run by the Royal Botanic Gardens at Kew - holds seeds from the planet's most endangered species along with a collection of native UK flora.

Together, they form the greatest concentration of biodiversity on Earth.

A Kew spokesman said: "Most seeds do not look very exciting but they are tiny miracles of packaging, containing all the genetic information for the next generation of plants."

Kew director Peter Crane said: "Wild plants are under greater threat of extinction than at any time since the last Ice Age."

The underground project, which is funded by £30 million from the Millennium Commission and £9.2 million from the Wellcome Trust, uses advanced technology to keep seeds in suspended animation..

The Prince, a keen supporter of conservation, was shown seeds through a microscope to see how they can be germinated. He also witnessed how precious seeds are dried to about only 10 per cent desert-level humidity and then deep-frozen at a bone-chilling minus 20 degrees Centigrade before being stored for hundreds of years.

Britain is the first country to have preserved its botanical heritage in this way.

Prince Charles launched the Millennium Seedbank Appeal in May 1996, with Sir David Attenborough as its Patron. The project was backed by a £30 million Lottery award from the Millennium Commission, more than £9 million from The Wellcome Trust - one of the world's largest medical research charities - and Orange plc, the Premier Sponsor of the Appeal.

Charles was also visiting another conservation project, at Basingstoke, to promote the coppice industry and create rural jobs.

Earlier today, the Prince of Wales was in Eastbourne, where he formally opened a new Age Concern centre. break from the serious business in hand.