The closure of Maltby Colliery in Rotherham, which is one of the country's last remaining deep coal mines, has moved a step closer after the owners proposed that the site be mothballed.
Hargreaves Services said Maltby Colliery, which has been producing coal for more than 100 years, was no longer viable on health and safety, geological and financial grounds.
The board is proposing that the pit, which employs around 500 workers, will be mothballed, putting all employees at risk of redundancy.
The company said it will continue to consult with unions and asked for any alternative proposals to be tabled by November 30.
"Although no viable alternative strategic options have been identified by the board at this time, as part of this ongoing consultative process the board will offer the opportunity to employees and trade union representatives to table alternative proposals," the company said in a statement.
Geological reports in recent weeks indicated that the risks associated with mining at Maltby had not significantly reduced, Hargreaves added.
Maltby produces more than a million tonnes of coal a year, with around 60% of its output supplied under a long-term contract with Drax, Europe's largest coal-fired power plant.
Much of the remaining production is sent to Hargreaves' Monckton works near Barnsley, which produces 200,000 tonnes of coke for use in the production of glass, detergents and steel manufacturing.
The company first disclosed the problems at Maltby in May, although mining has continued since then.