The NUM will sue Raleys Solicitors on behalf of members if a court finds the firm did not act in miners' best interests.
Regent Street-based Raleys has a long association with the union and was the recommended 'union solicitor' for men making compensation claims.
But in a hearing before a senior judge in Leeds the firm is named as one of several which allegedly failed to properly advise claimants. A handful of cases have been brought before the judge, Simon Hawkesworth, QC, to rule on medical issues.
It means that tens of thousands of men could sue their former solicitors. Many may have missed out by as much as £30,000, Judge Hawkesworth said.
If that is proven to be the case, the NUM would take action against Raleys, union secretary Chris Kitchen said yesterday.
A Raleys spokesman said: "We will continue to defend these claims. We remain committed to ensuring that all clients receive full compensation."
