Kellingley Colliery could close within 18 months, with the loss of around 700 jobs, a union has claimed.
UK Coal is consulting on plans to shut Kellingley, which employs 700 people, together with Thoresby in Nottinghamshire, which employs 600.
Chris Kitchen, National Union of Miners general secretary, said the union was told at a meeting last night the two sites would close by October 2015 at the latest, unless the government stepped in to support the firm.
He said: “We met with UK Coal last night and they issued statutory redundancy notices for every member of staff at Kellingley and Thoresby which means they will lose their job between now and October 2015.
“It’s devastating for the people that are going to lose their jobs and I think it’s devastating for the country too.”
Kellingley was saved from closure in July last year after pit bosses struck a deal with a pension and rescue fund from the Pension Protection Fund (PPF).
But Mr Kitchen added that the pits could close immediately if the PPF funding stops.
UK Coal, which also runs seven other pits across the country, said it hoped to secure at least £10m of investment within weeks but added that it would not guarantee the future of any jobs.
A UK Coal spokesman said: “One of the options is that Kellingley Colliery could close within 18 months. We expect that there will be some redundancies, whether or not we get the investment, but until we have finished the consultation nothing is certain.
“We will spend the next 45 days consulting and we don’t expect anything will change in that period.”