The government is to create a marine reserve almost as big as the UK in the Atlantic waters of Ascension Island.
Just over half of the protected area will be closed to fishing.
The fishery in the other half will be policed under a grant of £300,000 from the Louis Bacon Foundation, a charitable body.
It is the latest marine reserve to be declared around remote islands, which will increase marine conservation zones to about 2% of the ocean.
That remains a far cry from the 30% recommended by scientists to preserve species and expand fish stocks, but is much more than just a few years ago.
Governments have designated marine parks at Palau in the North Pacific, Easter Island and Pitcairn in the South Pacific, and New Zealand's Kermadec islands, in what has become a landmark year for ocean conservation.
The latest reserve at Ascension Island is said to hold some of the largest marlin in the world, one of the largest populations of green turtles, big colonies of tropical seabirds and the island's own unique frigate bird.
The reserve totals 234,291 sq km, slightly less than the size of the United Kingdom. It could be ready for formal designation as soon as 2017, once further data has been collected and analysed.
Dr Judith Brown, director of fisheries and marine conservation for Ascension Island government, said: "The economic benefit from the fishery has provided much-needed income for the island.
"This donation will help fund the enforcement to protect the closed area from illegal fishing."
The Great British Oceans Coalition, which includes the Blue Marine Foundation and the Royal Society for the Protection of Birds, has been campaigning since 2014 for the designation of all or part of Ascension's waters.
Charles Clover, Blue Marine Foundation chairman, said: "Ascension has been at the frontiers of science since Charles Darwin went there in the 19th Century, so it is entirely appropriate that it is now at the centre of a great scientific effort to design the Atlantic's largest marine reserve."
An accident of colonial history has left the UK and France with huge potential to safeguard marine life around remote oceanic islands.
The Conservative party promised in its 2015 manifesto to create a "blue belt" of protected ocean round the UK's Overseas Territories.
The commitment was described by Tory MP Zac Goldsmith as "the biggest conservation commitment by any government ever, pledging to protect an area of ocean three-and-a-half times the size of Britain".
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