Production has started on the massive Culzean gas field in the North Sea.
Operator Total said the field, 140 miles (225km) off Aberdeen, will be responsible for 5% of the UK's gas needs when it reaches peak production.
Culzean was discovered in 2008 when it was described as the largest gas find in a decade. It contains gas reserves equal to about 250-300 million barrels of oil.
The find was made by the Danish firm Maersk Oil.
Maersk was subsequently bought by Total.
The French firm owns a 49.99% stake, with the remainder shared between BP and JX Nippon.
Total's president of exploration and production, Arnaud Breuilla, said: "Culzean is a good example of our efforts to upgrade our portfolio in the North Sea over the last years, notably by bringing Total and Maersk Oil together.
"The Culzean field is located in the Central Graben area, close to the Elgin-Franklin fields, also operated by Total.
"The Culzean project is delivered ahead of schedule and more than 10% below the initial budget, which represents capex (capital expenditure) savings of more than 500 million dollars."
About 100,000 barrels of oil-equivalent gas per day will come from Culzean via a pipeline to Teesside.
It consists of three bridge-linked installations and a floating gas storage facility.