While thousands of former and current coal miners have joined a class action against companies regarding occupational lung diseases, spearheaded by human rights lawyer Richard Spoor, his law firm says the companies have signalled they plan to oppose the certification application.

Richard Spoor Incorporated (RSI) said on Tuesday that its class action against coal companies had been bolstered by thousands of former and current mine workers joining its ranks, and it estimated that hundreds of thousands could eventually take part.

RSI spearheaded a historic R5-billion settlement on behalf of gold miners who contracted the incurable lung disease silicosis from inhaling silica dust.

Last year, RSI filed papers against South32 Group, BHP Billiton, Seriti Power, Exxaro Group, the Anglo American Group and Glencore, alleging they had not taken the necessary precautions to protect workers from coal mine dust lung diseases (CMDLD) such as pneumoconiosis and chronic obstructive pulmonary disease.

The companies have both former and current coal assets in South Africa.

"In our historic R5-billion settlement in the silicosis case, an estimated 15,000 to 500,000 gold miners were part of the class action. If the coal class action follows a similar trajectory, the number of affected workers could potentially be in the hundreds of thousands again," RSI director George Kahn said in a statement.

A class action in South Africa follows a two-stage process. The first is an application to have the class certified by a court.

"At this point, all parties...