Under the permit, formation water to be injected at Singleton can come from the site itself and other oil wells in southern England. Dumping has been going on at Singleton since 1996. It was part of a national review of permitting at older oil and gas sites under environmental regulations first introduced in 2013.

The EA said: “We are satisfied that the permit will ensure that a high level of protection is provided for the environment and human health and that the activities will not give rise to any significant pollution of the environment or harm to human health.”

The liquid waste would be injected to a depth of about 800m into the Ashdown formation, layers of sandstones, siltstones and mudstones.

The EA said water in the Ashdown formation was saline and unsuitable for agriculture, industrial supply or drinking.

It would not support springs, watercourses or wetlands and the formation could not be used for geothermal energy, the EA said.

Opponents of onshore oil and gas operations raised concerns about IGas’s plans during a public consultation last year. The Chichester area is heavily faulted and has been the centre of numerous small natural earth tremors, dating back to the 18th century. They also said the operation threatened water quality in the chalk aquifer.

In response, the EA said: “There is no immediate apparent viable pathways to shallower aquifers. “We are therefore satisfied that the proposed discharge into the Ashdown formation will not impact on the shallower aquifers.”

The South Downs National Park Authority, which handles planning applications in the area, recently told IGas it did not have planning permission to import liquid waste from other sites for disposal at Singleton. The company has said it does “not necessarily agree”.

IGas said there had been no changes to the operations at Singleton or to the composition of the water. Singleton was the UK’s second largest onshore oil producer in 2020, based on data from the Oil and Gas Authority. It extracted 2.4 per cent of UK onshore oil, behind the top producer, Wytch Farm, on 84.3 per cent.

The water cut – the proportion of formation water in the total fluid volume produced at a site – tends to increase as wells age.