LONDON - Britain's approval of its first new deep coal mine in decades was unlawful because ministers failed to consider the "adverse international signal" sent by the controversial decision, environmental campaigners told London's High Court on Tuesday.

Friends of the Earth and South Lakeland Action on Climate Change are challenging the previous Conservative government's 2022 approval of the coking coal mine in northwest England.

Lawyers representing SLACC said in court filings that ministers did not consider the impact on Britain's status as a climate leader and the potential effect of "encouraging other countries to permit new fossil fuel developments".

Britain dropped its defence of the legal challenges after a Supreme Court ruling which said planning authorities must consider the impact of burning, not just extracting, fossil fuels when deciding whether to approve projects.

But developer West Cumbria Mining is still fighting the case and says the project – which plans to extract coking coal for manufacturing steel, rather than to generate electricity – would be "a unique 'net zero' mine".

West Cumbria Mining's lawyer James Strachan said in court filings that the development would not cause a net increase in greenhouse gas emissions, as the use of coking coal extracted from the mine is driven by demand for steel.

Strachan also argued that the approval of the mine would be positive for Britain's global standing, as it would meet a need for coking coal extracted from within the country.

Lawyers representing the campaign groups, however, argued the international impact of approving the mine was central to the planning dispute and had been overlooked by ministers.

Friends of the Earth's lawyer Paul Brown said in court filings that the issue "was not what kind of climate virtue signalling the UK should engage in", but whether approving the mine would diminish Britain's ability to influence international negotiations.

The hearing is due to finish on Thursday, with a ruling expected at a later date.