The announcement means more uncertainty for ATH's 300 workers, with the majority based at mines in Ayrshire, Fife and Dumfries & Galloway.
The shares ended the day down 11.78% at 0.38p, having been trading at greater than 40p at the start of this year.
In a statement ATH, headed by chief executive Alistair Black, confirmed it was in the early stages of working with KPMG but said: "The directors believe that in the eventuality that a transaction is undertaken, it is unlikely existing shareholder value will be maintained."
ATH, which has been struggling due to the plunging price of coal, had been working with Deloitte for several weeks on a sale or restructuring.
But last week ATH's debt was bought from Clydesdale Bank and HSBC by a fund run by venture capital firm Better Capital which led to a separate review being launched.
Jon Moulton, founder of Better Capital, previously ran Alchemy Partners which was the largest shareholder in ATH when it floated on AIM in 2004.