The reserve will be based around the remote Pitcairn Islands archipelago, a British overseas territory that is inhabited by descendants of the sailors who staged a famous mutiny on the Bounty ship in 1789.
"The government intends to proceed with designation of a MPA (Marine Protected Area) around Pitcairn," read the budget unveiled by finance minister George Osborne in parliament.
But the government said a final deal would depend on agreements for satellite monitoring of the vast area, preventing ports from landing illegal fish catches and naval patrols.
It is home to at least 1,249 species of marine mammals, seabirds and fish and includes the world's deepest known living plant -- a species of encrusting coralline algae found at a depth of 382 metres.
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"The new reserve protects some of the most near-pristine ocean habitat on Earth," Pew said in a statement.
In 2013, Pew, National Geographic and the local elected body on the remote archipelago, the Pitcairn Island Council, submitted a proposal for the creation of the reserve.
Pitcairn was settled in 1789 by mutineers from the British naval ship the Bounty, who famously set their captain William Bligh adrift in the South Pacific.
Many of the families of the mutineers moved from Pitcairn, a five-square-kilometre island midway between New Zealand and Chile, to the larger Norfolk Island in 1856.
Enric Sala, National Geographic's explorer-in-residence, a member of a scientific expedition that visited the islands, said the move "will protect the true bounty of the Pitcairn Islands -- the array of unique marine life in the surrounding pristine seas."