The banking of the billionth seed at the Millennium Seed Bank is being celebrated at a ceremony to mark International Biodiversity Day.
The Millennium Seed Bank already contains 18,000 species
Worldwide partners in the project will be joined by biodiversity minister Barry Gardiner at Wakehurst Place, in Ardingly, West Sussex.
The billionth seed, from an African bamboo, joined 18,000 other wild plant species stored at the site last month.
The seed bank holds the largest wild seed collection in the world.
Tuesday's official ceremony will be attended by ambassadors from Millennium Seed Bank Project partner countries, including representatives from Tanzania, Malawi, Madagascar and China.
The seed bank is owned and managed by the Royal Botanic Gardens at Kew, and was founded in the 1990s with the intention of protecting plants from extinction.
Several thousand seeds of each species are kept at the Sussex site.
By 2010, teams of scientists and technicians there will have banked 10% of the world's wild flowering plants.
They are aiming to have collected 25% by 2020, but that is dependant on whether more funding can be secured.
The Millennium Seed Bank has the capacity to store up to half the world's wild flowering plant species and already holds 88% of the UK's flora, including more than 75% of its threatened plant species.