Located 150km off the coast of Lincolnshire, the field should contribute 5% of UK gas production and supply enough gas to heat the equivalent of 1.5 million UK homes at its peak, and become the largest producing gas field in the North Sea in 2017.
The project - a partnership between Engie E&P. Centrica and Bayerngas - has been in development for five years, during which time bosses estimate it has added £1.3bn to the UK economy and supported nearly 5,000 job.
Ruud Zoon, managing director of Engie E&P UK Limited, said: "First gas from Cygnus represents a significant boost to the UK's long term energy security, with an estimated production life exceeding 20 years.
"Furthermore, with over 80% of the contract work secured by British businesses - including fabrication yards in Hartlepool, Methil and Burntisland - it is a showcase for the technical and commercial excellence of the UK oil and gas supply chain."
The gas has come from the Cygnus Alpha platform but work is already under way to extend the field further via fresh wells being drilled at the Bravo platform.
The complex includes four platforms, 10 wells and two subsea structures, serving an estimated field size of 250 square kilometres, which it is expected will achieve a consistent production volume of 250m cubic feet per day.
Gerry Harrison, Bayerngas UK managing director, said: "ENGIE E&P, Centrica and Bayerngas have worked closely together on this project since 2010. As you will hear from the partnership, most of the numbers related to this project are very large, for instance: investment, man hours, production levels, reserves, longevity. But perhaps the most important numbers are very small; our priority has always been safety and I am proud that Bayerngas has collaborated on a project with minimal lost time incidents."
Chris Cox, managing director of Centrica's exploration and production business, said: "The Cygnus field is hugely important to the country's energy supply, so we are proud that gas is now flowing from the field and into homes and businesses across the UK. This milestone was only possible thanks to the hard work and collaboration of the teams across Centrica, ENGIE E&P, Bayerngas and our supply chain partners."