Retired firefighters who were responsible for tackling blazes on military bases have been unable to access their full pension due to “numerous” blunders by the outsourcing group Capita, it has emerged.

Capita won a £525m contract to run the Ministry of Defence’s fire and rescue service in 2019, renaming it the Defence Fire and Rescue Project (DFRP) after the privatisation.

Since then, some retired workers from the service and spouses of deceased pensioners have complained that their benefits have been miscalculated, reducing their payments, while others have been left waiting up to eight months for money they were owed.

The trade union Unite said problems with the scheme raised concerns about Capita’s upcoming contract to take on the administration of the entire civil service pension scheme, starting from 2025.

The mistakes affect some retired members of a service that fights fires on military bases in the UK and overseas, using specialised equipment in particularly hazardous conditions, such as in proximity to ammunition or other military hardware.

The DFRP scheme is not administered by Capita but by MyCSP, part of another outsourcing company called Equiniti.

Capita admitted that it had supplied Equiniti with incorrect employment data about some of the scheme’s 700 members, an error that is understood to be at the root of delays and miscalculations affecting payouts.

In a letter to the trade union Unite, seen by the Guardian, the Cabinet Office acknowledged that Capita “may not have provided the accurate data”, adding that it expects any issues to be fixed by July 2024.

Members of the pension scheme told the Guardian that they were still owed hundreds of pounds or had waited months for any payment at all.

Frank Gallacher paid into his pension pot for more than 40 years, and submitted his pension forms in November 2022, in time for his preferred retirement date of February 2023.

The former worker at the Royal Navy’s Clyde Nuclear Submarine Base chased payment for four months and was eventually told that he would receive benefits from June 2023 but this did not happen until August, after multiple complaints by Unite.

“This was the most stressful episode of my life,” said Gallacher, who worked for the MoD fire and rescue service for five decades. “I can only assume the transfer from the MoD to Capita Fire Rescue for some reason made my pension disappear.”

Sally Stringer said she had missed out on a higher rate associated with a promotion that her late husband had won. “This only added to the trauma and loss I already felt,” she said.

Unite said it believed “numerous” scheme members have been affected and that some may not have come forward.

The union said the mistakes raised concerns about the Cabinet Office’s recent award of a 10-year contract to run the civil service pension scheme, which has 1.6 million members, to Capita.

Unite’s general secretary, Sharon Graham, said: “It beggars belief to think that Capita, a company that has been bungling the management of pensions for 700 hard working firefighters, has now been rewarded with a green light to take over running all the pensions in the civil service next year.

“Once again, this demonstrates what happens when outsourcing and cost-cutting are valued above people’s lives.”

The Guardian understands that Capita has performed multiple internal audits to find out why it submitted incomplete employee data to MyCSP.

However, the company insisted it was not to blame for all of the pension problems.

“We do not administer DFRP pensions, but we provide payroll data to the administrator, [MyCSP]” a Capita spokesperson said. “We have worked with Unite to review and resubmit payroll data to the administrator to ensure they are able to make accurate payments. This review has not prevented pension payments being made.”

MyCSP declined to comment.

Capita is due to take over administration of the scheme from MyCSP in 2025 as part of its wider civil service pensions contract.