The Surrey County Council was ordered by the Supreme Court to take the effects of burning oil from newly drilled wells in Horley on the climate into account.
It has always been assumed by planning law that the only factors to be taken into account are those resulting from the construction of the wells, rather than how the finished oil products will be used.
Campaigners' lawsuit, filed by Sarah Finch, may jeopardize upcoming fossil fuel projects in the UK.
The council said it followed planning law.
Today’s judgement relates to the Horse Hill oil well in Surrey. The site was given planning permission to expand its operations in 2019, a decision that was challenged by a local resident, Sarah Finch, on behalf of Weald Action Group.
Lawyers acting for Ms Finch said that as part of the Environmental Impact Assessment the council should have considered not just the greenhouse gas emissions from building the wells but also burning any oil that was then used – known as downstream emissions.
The six wells at Horse Hill are expected to produce 3.3 million tonnes of crude oil over the next 20 years. Burning that oil would produce over 10 million tonnes of CO2 the environmental campaigning organisation Friends of the Earth has estimated.
The judgement - by a three-to-two majority – is likely to have a significant impact on decisions on future fossil fuel projects in the UK.
Campaigners claim the judgement could also effect ongoing legal challenges of new projects in the UK including Rosebank oil field and the proposed coal mine in Whitehaven in Cumbria too.
Because the UK law on environmental assessments is based in part on European law the ruling could have impacts abroad as well, they say.