The vault is being built on land adjoining Kew's sister garden at Wakehurst Place in Sussex. Here seeds from 25,000 species representing some 10 per cent of the world's estimated 250,000 flowering plants grown in the wild will be stored in containers at temperatures up to minus 40 degrees centigrade in a state of suspended animation for 200 years or more. Thus samples of threatened species which serve humankind as sources of food, clothing, fuel, fibres and medicines can be germinated by future generations for reproduction and use in development projects and scientific research.

A grant of up to 30 million pounds sterling has been awarded from the national lottery, and further support for the initiative is coming from corporate and individual donors responding to an appeal launched by the Prince of Wales and the television naturalist Sir David Attenborough.

Describing it as one of the first conservation projects to meet the problem of species extinction on an appropriate scale, Sir David said scientists believed that up to 25 per cent of the world's plants risked extinction in the next 50 years. Many grew on the fringes of deserts where a quarter of the world population depended upon them for food, medicine and shelter.

Kew's existing seed bank at the Wakehurst Place garden is regarded by botanists as the most comprehensive in the world at present, even though it represents less than two per cent of global flora. The decision to expand the bank's role was taken in the light of Britain's signing at the Rio conference in 1992 of the Convention on Biological Diversity. It followed a study of the scope of existing seed banks which were found to be mainly devoted to crop rather than endangered wild varieties.

Meanwhile Kew has stepped up its continuing collection of world seeds in partnership with local botanists in Saudi Arabia, Oman, South Africa, Jordan, Yemen, and Tunisia. Interest has been shown by Bahrain, the United Arab Emirates and Kuwait, which is linked with Kew through the Institute for Scientific Research which receives regular lists of seed samples available from the existing bank at Wakehurst Place.

Botanists in Kuwait have participated in compiling the Flora of the Arabian Peninsula and Socotra. Coordinated by Kew and the Edinburgh Botanic Gardens, the first of an envisaged six volumes was published two years ago.

The bank receives daily seed requests from botany-related organisations in Britain and abroad. About one in three of every species held has been put to good use in such projects as the search for a Parkinson's disease cure using vetchlings, identifying chemicals in a tropical weed Bidens Pilosa for treating ringworm and reafforestation in Uganda where trees grown from seeds provided are helping to halt erosion and provide fuel, building materials and fodder to local people.

Major collecting worldwide over a period of ten years for the new Millennium Bank will not begin until the year 2000 and capacity is available. Where possible up to half of the seeds gathered will be kept by the countries concerned. For those without seed banks the entire collection can be stored at Wakehurst Place until a bank is established in the country of origin.

Much of the seed processing and research at the new bank will be carried out by visiting scientists on the flora of their own countries. Tourists visiting Wakehurst Place, currently some 250,000 annually, will be able to watch them at work through the central atrium on the ground above the vault where interactive exhibits will explain the vital role of seed conservation in combating the destruction of the natural environment.

The main focus of the collection will be on flora of the world's tropical dryland areas which support people who depend upon plants for stabilising and improving soils, as well as the products they yield. Plants used in hedging, land stabilisation and medicine will be among those expected to be sought from the Arabian peninsula.

Plants from arid and semi-arid regions produce seeds that are particularly suited to long-term storage because they are naturally adapted to germinate after long periods of drought. One of the best things about the project is that the technology involved in drying, freezing and storing for future germination is already proven.

Preceding the international collection, volunteers from British research bodies and conservation organisations are gathering seeds from all of Britain's 1,400 or so native plants. The completed collection, the first made by a country to conserve its entire flora, will be transferred to the new Bank, which is Kew's gift to the world, thanks to sponsors, including the British people who buy lottery tickets in the hope of becoming millionaires.