LONDON, Sept 27 (Reuters) - Britain has given the go-ahead for one of its biggest new oil and gas projects in years, Equinor's (EQNR.OL) North Sea Rosebank field, saying energy security was the priority despite opposition from environmentalists.

Wednesday's announcement comes after Prime Minister Rishi Sunak watered down interim plans for the government's 2050 net zero emissions target, a move that critics said could also encourage other countries to rein in their climate ambitions.

Norwegian energy group Equinor said the earliest the field, located west of the Shetland Islands and due to start output in 2026/27, would be electrified is 2030.

Equinor said the type of light, sweet oil contained in Rosebank can be used in most refineries, including in Britain, and that the country can obtain the crude via the open market.

Equinor, which holds a majority stake in Rosebank will invest $3.8 billion alongside its partner Ithaca Energy (ITH.L) to develop the first phase of the field.

The development approval follows warnings from many North Sea producers, including Ithaca and Equinor, that a windfall tax the British government imposed on the industry in the wake of the 2022 energy price shock would deter investment in the basin.

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