Production is to end at one of the UK's few remaining deep coal mines with the loss of more than 400 jobs.
Harworth is one of the deepest mines in the UK
Work is continuing on one seam at Harworth colliery on the Yorkshire and Nottinghamshire border but once that is finished the mine will be mothballed.
UK Coal blamed geological problems and said it hoped most of the workers will be able to transfer to other pits.
The firm added that substantial coal remains at Harworth which it hopes to mine in the future.
A UK Coal spokesman said geological problems on the face being worked had resulted in "significant losses".
Dave McGarry, of the coal mining union UDM, said the workers had been sent letters outlining their options to transfer to another site or take redundancy.
"It is difficult because you're asking a man to make a decision on his future in three or four days. It is hard times."
A target of mining 30,000 tonnes a week from the seam has been set with production ending once the remaining million tonnes has been extracted.
However, if production cannot be improved to the 30,000 target, the pit will close earlier.
Harworth employs 460 people who will be offered the chance to move to the UK Coal pits at Welbeck and Thorsesby in Nottinghamshire, Kellingley in North Yorkshire or Daw Mill in Warwickshire.
Only a handful of workers will remain once the pit is mothballed.
The future of Haworth depends on being able to access another seam of coal where geological conditions are thought to be better, the UK Coal spokesman added.
But this would require help from the government towards the £50m it is estimated it would cost to open the new face.
Harworth Colliery is in the village of Bircotes nine miles south of Doncaster and has been producing coal for more than 80 years.
Most of its output goes to power stations in the Trent Valley.