The Mariner field is expected to produce more than 300 million barrels of oil over the next 30 years.
Its Norwegian operator Equinor said the development will support more than 700 long-term jobs and generate significant revenue in the supply chain for decades to come.
Mariner is expected to produce annual average plateau rates of around 55,000 barrels of oil per day and up to 70,000 barrels of oil per day at peak production.
WWF Scotland said the announcement is bad news for efforts to tackle the climate emergency and the vast majority of known fossil fuel reserves should be left in the ground.
Anders Opedal, executive vice-president for technology, projects and drilling at Equinor, said: “By gathering and interpreting new seismic data we have improved our understanding of the reservoirs. This has resulted in fewer and better placed wells and increased resources since the project was sanctioned in 2012.With the significant volumes in place, we see clear potential to further increase the oil recovery from the Mariner field and will proactively seek opportunities to do so through the application of new technology, additional drilling and future tie back opportunities.”
He added: “With the start-up of Mariner, we have delivered one of the most complex developments in the North Sea and Equinor’s portfolio.
“We will continue to apply digital solutions and new technology to deliver safe and efficient operations and optimize production.”
Mariner is located on the East Shetland Platform of the UK North Sea, around 95 miles east of Shetland and 198 miles north-east of Aberdeen.
It was discovered in 1981 but was left untouched until 2007, by which time technology had advanced sufficiently to develop it.
Robin Parker, climate and energy policy manager at WWF Scotland, said: “Oil flowing from this new oil well is bad news for efforts to tackle the climate emergency.
“We’ve already moved away from burning fossil fuels to generate electricity, and dealing with the climate crisis means doing the same for how we heat our homes and travel around.
“Our economic future is not in fossil fuels, we need to see a just transition that harnesses the engineering skills currently deployed in the oil and gas industry and applies them to the climate-friendly economy we are building.”