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By Hywel Griffith
BBC News Wales correspondent
The operator of the UK's last opencast coal mine has been accused of breaching its licence after inspectors found it was mining outside its permitted area.
Merthyr Tydfil's Ffos-y-Fran mine produces two-thirds of the UK's coal and was supposed to close in September as its planning permission had expired.
Operator Merthyr (South Wales) Ltd said it would not be appropriate to comment "while the appeal process is ongoing".
The owner was issued with a local authority enforcement order to stop digging earlier this year, meaning production must cease by the end of July, but the company appealed against this decision.
The Coal Authority, which issues licences in the UK, has now sent the company a final enforcement notice, saying it is satisfied the operator is continuing to mine in contravention of the 1994 Coal Industry Act.
It stated: "You are required to cease all extraction of coal outside of the licence area with immediate effect and inform the authority that this has taken place."
Merthyr (South Wales) Ltd wanted to extend the licence until 2024, arguing coal from the mine was needed by the steel industry.
But council planning officials refused the application in April, saying the proposed extension did not fit with Welsh government policies on tackling climate change.
It means production is set to end at Ffos-y-Fran after 16 years of excavation.
It originally won planning permission in 2005 and work began two years later to excavate 11 million tonnes of coal across a site the size of 400 football pitches.
The other aim was to restore the land - riddled with the remains of old industries - back to green hillside for the community's benefit as work progressed.
But there was stiff opposition due to the mine's proximity to homes and businesses.
The closest houses were initially less than 40m (132ft) away, and residents led a long campaign, saying their lives were being blighted by coal dust and noise.
Merthyr (South Wales) Ltd said: "Having previously advised it would not be appropriate to comment whilst the appeal process is ongoing, we will not be responding to any further requests from the BBC."
The Welsh government said making any comment while the appeal process was ongoing could "jeopardise any future decision Welsh ministers may have to make on the matter".
Merthyr Tydfil council said its planning division "continue to review the appropriate enforcement options in light of the continued operations taking place, which includes the use of a stop notice".
It added: "We are aware of the steps being taken by the Coal Authority and we will continue to monitor the situation."