About 17,000 ex-mineworkers were wrongly told they owed thousands of pounds in tax due to an error.
One of them was 79-year-old Michael Hinchley, from Nottinghamshire, who said he was shocked after being told in a letter he owed £26,000.
Capita, the administrator for the Mineworkers’ Pension Scheme, said the tax code letters were sent by mistake.
The National Union of Mineworkers said it showed a lack of “human contact and concern” for UK miners.
Michael Hinchley, 79, from Mansfield Woodhouse, a former electrician at Mansfield and Sherwood collieries, received the letter, stating he owed £26,235.36, on Monday.
He said: “I thought ‘how can I owe that when I’ve been paying tax on my pension?’
“If someone who is elderly and doesn’t have family to help them out gets a letter like that it could be really worrying for them.”
Chris Kitchen, general secretary of the National Union of Mineworkers (NUM), said the union was “inundated” with calls after 17,000 former miners were issued with the letters, with some told they owed £50,000 in tax.
He said the union was concerned no-one at the Her Majesty’s Revenue and Customs (HMRC) picked up on the error before the letters were generated.
“The letter would have been concerning for pensioners who already have small pensions and haven’t got a large amount of money,” he said.
“I don’t want them to get distressed over it… it’s an error in the new system.
“But it’s highlighted our concerns of whether Capita are up to the job – there’s a lack of human contact and concern with HMRC, too.”
The Mineworkers’ Pension Scheme said in an automated response to callers that the correspondence received from HMRC was issued in error and it was working hard to rectify it.
HMRC said it was “sorry” that Mr Hinchley received a “coding notice in error”.
A spokesman said: “Where we are provided with incorrect information, we work hard to put matters right.
“We will ensure that no customer will lose out as a result of this.”
A Capita spokesperson said: “Following a change to HM Revenue & Customs’ online PAYE portal, an incorrect update was made in processing information for members of the Mineworkers Pension scheme.
“This issue has been resolved and all members affected will shortly receive letters to advise that they do not need to take any action.
“We sincerely apologise for any concern and inconvenience this has caused.”