After opponents have been granted a judicial hearing, a legal challenge to mine expansion plans will be filed.
January saw approval for 40 million tonnes more coal being dug at Aberpergwm and Neath Port Talbot.
Campaigners stated at the time that they were contemplating legal action.
The Welsh government declined to comment, citing legal proceedings. In the past, the Coal Authority was to be restructured following a dispute over the plans.
Coal Action Network, a protest group, has stated that it is now preparing to go before the court.
It tweeted: "We don’t know how we can give up because there are so many things to fight for."
The Welsh Liberal Democrats backed the action.
Jane Dodds, the leader of the party, called for the Welsh government "say no more coal" and accused the government of being slow in approving the Aberpergwm expansion.
She stated that the expansion of the mine must be stopped.
Ms. Dodds stated that, "while some will point out the jobs it creates," Aberpergwm communities should be given a leading part in the development and implementation of the green hydrogen strategy in south Wales. This will ensure a just transition for communities and jobs that will last a long time rather than a decade.
Ministers from the Welsh government have stated that the Coal Authority's responsibilities, which is managed by the UK government, conflict with UK's climate change commitments.
