ATH, which operates mines in Ayrshire, Dumfries and Galloway and Fife, is now worth just £2 million after its shares closed at a new low of 5p, down 4.5p or 47.4% on the day.
This is less than one-eighth of the company's value at the start of the year when its shares were trading at 42p.
The company also said it had lost further reserves at its Muir Dean mine in Fife, where it has been hampered by old workings.
The news comes just weeks after ATH, Scotland's second largest coal producer after Scottish Coal, halted an extension to its Glenmuckloch mine in Dumfries and Galloway, as it slashed capital expenditure. It warned yesterday that some site openings would also be delayed.
The company, which also operates Skares, Duncanziemere and Netherton mines in Ayrshire, said coal prices had dropped by one-fifth this year.
Meanwhile, it has been told by the Environment Agency that it will have to purchase £1.1m of carbon credits in each of July 2012 and July 2013 to cover its conveyor belt at the Glenmuckloch mine as part of the Government's Carbon Reduction Commitment Scheme.
ATH, which employs around 300 people in Scotland, said this meant its trading performance would be "materially adversely impacted".
Chief executive Alistair Black told investors: "The board continues to have discussions with its banking syndicate, primarily driven by external factors such as the downward movement in coal prices and the Carbon Reduction Commitment Scheme, in order to secure adequate facilities."
ATH's loan deal with HSBC and Yorkshire Bank, owned by Clydesdale Bank, was extended in September but is due to expire in May 2013. It had net borrowings of £31.5m at the end of its financial year in October.
In a trading update published yesterday, ATH said: "International coal prices have fallen by some 21% since the beginning of the calendar year and by over 28% since the beginning of the group's financial year. While ATH has been able to successfully renegotiate a significant improvement to its two remaining legacy contracts and enter into some new fixed-price contracts at prices well above those currently available, further reserve losses in recent weeks at Muir Dean, together with those previously announced in March due to increased old workings, coupled with reduced demand and pricing of its premium products have exceeded any gains made."
The fall in prices has been blamed on oversupply of coal in North America due to a mild winter and power companies turning to cheap fuel from the rapidly expanding shale gas sector.
ATH said: "The ongoing weakening of international coal prices continues to adversely impact group income and with commodity markets forecasting this trend is set to continue, the group is reviewing its mining plans to maximise cash generation through this difficult period.
"New sites will continue to be developed through the planning process; however, the opening of some sites may be delayed and capital expenditure kept to a minimum while coal prices remain subdued."
The company said this would not affect its restoration of closing sites. A spokesman said: "The company remains wholly committed to meeting its restoration obligations for its sites. That remains the case and will continue to be the case while it reviews its mining plans."
ATH is furious that its 12.2 km conveyor belt that transports coal from Glenmuckloch to its Crowbandsgate rail facility near New Cumnock has been caught by the Carbon Reduction Commitment Scheme, arguing that it has saved tens of thousands of lorry journeys. It has lodged an application for a judicial review and said the £1.1m a year cost of the scheme could be reclaimed if its appeal is successful. It is also seeking legal advice about whether it can ask the courts to postpone its carbon credits purchase.
ATH, which is headquartered in Doncaster, is due to publish its results for the six months to April 1 at the end of this month.