The RMS St Helena anchored in the bay of the South Atlantic island of Saint Helena

Britain on Thursday announced plans to double the area of ocean under marine protection around the country's overseas territories to a size greater than the landmass of India.

Fully protected marine reserves are to be set up around the Pitcairn Islands in the South Pacific and St Helena, Tristan da Cunha and Ascension islands in the South Atlantic.

The plans impose a permanent ban on commercial fishing in an additional one million square kilometres (386,100 square miles) of , preserving important marine life, according to the Foreign Office.

Britain will also provide £20 million (23.5 million euros, $26.4 million) over the next four years to fund the implementation, management, surveillance and enforcement of the new Marine Protected Areas (MPAs).

"Protecting four million square kilometres of ocean is a fantastic achievement, converting our historic legacy into modern environmental success," said Foreign Office minister Alan Duncan.

The announcement came shortly before US Secretary of State John Kerry was due to kick off the two-day "Our Ocean" conference in Washington, which will bring together governments, scientists, business leaders and NGOs from around the world.

Britain's plans include the designation of around St. Helena and Pitcairn and a commitment to designate marine protection zones around Ascension by 2019 and Tristan da Cunha by 2020.

The ocean around St. Helena is home to a wide range of marine life including whale sharks, humpbacks and turtles while Ascension Island hosts the largest green turtle rookery in the South Atlantic.

Tristan da Cunha will boast the Atlantic's largest MPA in the South Atlantic, protecting the Northern Rockhopper Penguin and the Tristan Albatross.