A new management plan for St Kilda has been signed to secure the future of the UK's only natural and cultural World Heritage Site, on the 82nd anniversary of the remote archipelago's evacuation.

St Kilda is one of only 27 locations to have been awarded mixed World Heritage Site status by Unesco, and the new agreement sets out how its unique character and history are to be managed and conserved over the next three decades.

The signatories are conservation charity the National Trust for Scotland, which owns and manages St Kilda on behalf of the nation, the local authority for the Western Isles, the Scottish Government agencies Historic Scotland and Scottish Natural Heritage, and the Ministry of Defence, which maintains a facility on St Kilda.

St Kilda is comprised of different islands – Hirta, Dun, Soay, Boreray and Levenish.

Previously it was thought Hirta alone was populated and that Boreray was only visited by islanders to hunt seabirds and gather wool from sheep. But last year archaeological evidence of a permanent human settlement was found there.

The new St Kilda World Heritage Site Management Plan describes the cultural and natural heritage of the remotest outpost of the British Isles.

St Kilda lies 41 miles west of Benbecula, where the agreement was signed last night.

The last 36 islanders were evacuated in 1930 and St Kilda was bequeathed to the National Trust for Scotland in 1957.

It was allocated World Heritage status by Unesco in 1986 and in July 2004 this was extended to include the surrounding marine environment.

In 2005, recognition was also given to St Kilda's unique cultural landscape.

The new plan, which establishes a framework for St Kilda's long-term management and conservation, deals with a range of threats to its unique habitats, including the risks posed by climate change to large colonies of gannets, northern fulmar, Leach's storm-petrels and puffins.