Raleys handled 60,000 claims for compensation from ex-miners
Six solicitors are facing a tribunal over allegations of misconduct relating to miners' compensation claims.
They are all partners at Raleys in Barnsley which handled 60,000 claims by ex-miners with work-related illnesses.
It is alleged there was a conflict of interest in dealings with the NUM which lent the firm £4m.
Last month, Doncaster solicitors James Beresford and Douglas Smith were struck off for serious professional misconduct in relation to miners' compensation.
Beresford, 58, said last year to be Britain's highest-earning solicitor, and Smith, 52, made millions of pounds from personal injury claims for miners under the government's coal health compensation scheme.
The Solicitors' Disciplinary Tribunal heard the men acted "unacceptably" by charging clients even though the government was paying their fees.
Both firms processed claims from former miners now suffering from illnesses such as lung conditions and vibration white finger.
On Tuesday, the tribunal was hearing the first day of evidence relating to Raleys.
The solicitors facing the allegations are Derek Barber, 44, Derek Firth, 50, Carol Gill, 38, James Gladman, 49, Jonathan Markham, 46, and Katherine Richards, 40.
It is alleged that their relationship with the NUM "compromised their independence and integrity and duty to act in the best interests of clients pursuing miners' compensation claims".
It is also alleged that "they failed properly to advise their clients" about the costs of claims.