UK Coal, the country's largest coal producer, has said it will make a loss this year due to production disruptions at several of its mines.
UK Coal is developing more surface mines
The firm's finances have improved in recent years and it made a £7m profit in the first six months of 2006.
But reduced output at four deep cast mines over the summer will force it into the red for the year as a whole.
The Doncaster-based firm wants to cut its reliance on coal, and got approval for its first wind farm last year.
It is also developing its substantial land assets as part of its plans to diversify its operations.
UK Coal supplies 7% of the country's electricity needs and employs 4,000 staff.
It operates deep cast mines in Northumberland, Yorkshire, Nottinghamshire and Warwickshire.
In a trading statement, it said disruption at four of these mines over the past three months had resulted in 422,000 tonnes of output being lost and the shortfall was not likely to be recovered in the final quarter.
But the firm said it was confident about its future prospects.
"UK Coal remains on track to deliver substantial value to shareholders.
"The group continues to make good progress towards planning approvals at a number of our key property sites and the deep mines will offer a significantly improved financial profile over the next 15 months."