Law firms which have charged miners and their families for handling compensation claims should give the money back, according to Wales Office minister Don Touhig.
Around £400m has been given out in compensation in Wales
The Islwyn MP, who chairs the Welsh group monitoring miners' compensation claims, said some solicitors have made thousands of pounds, even though the UK Government is paying all legal costs.
Mr Touhig said that, although the practice was not illegal, it was "wholly immoral".
However, the professional body for solicitors, the Law Society, said if people complained about this issue, it could only check whether solicitors were complying with the rules.
Last month, figures showed almost £400m has been paid to ex-miners and their families in Wales whose health suffered as a result of working underground.
A total of £284m has been paid out for respiratory claims and another £103m for vibration white finger, which is caused by the continuous use of vibrating hand machinery.
The chest disease compensation scheme ends on 31 March.
Mr Touhig stressed that all money awarded should be going directly to miners and their families.
"Every penny paid out should go directly to the claimant," he explained.
He said "a number of miners and their widows" had been persuaded to pay some part of their compensation to solicitors or other organisations for handling their claims.
"The government is handling all the legal costs - there isn't any justification for charging any miner or miner's widow for handling the case.
"In Wales, in the main we are very lucky. We have some good and very reputable solicitors who have done a fantastic job on behalf of miners and their widows in sorting out their claims."
The deadline for compensation for ex-miners passes in March
Mr Touhig urged people to get their claims in quickly, and to seek advice if they are not sure how to apply.
"But if a solicitor, or any other organisation, asks for money to handle the case, they shouldn't touch them with a bargepole," he said.
Geoffrey Negus, from the Law Society, said that, if people complained about this issue, it could only check whether solicitors are complying with professional rules.
"We are looking at whether claimants were told at the outset whether there would be deductions, and whether the deductions are reasonable," he said.
He said around 40 cases, mainly from the north of England, were currently being investigated, but more were expected.
It is almost six years since a landmark legal ruling opened the way for miners and their relatives to claim compensation for chest diseases suffered as a result of working in the pits.