Families and campaigners have demanded a public inquiry after a government watchdog found that at least 69 suicides could have been linked to problems with benefit claims over the last six years.

Amid anger from the loved ones of some of those who took their lives in circumstances where difficulties with the Department of Work and Pensions played a role, the National Audit Office (NAO) report said that the true number of deaths linked to claims could be far higher.

And it said that until recently the DWP had failed to actively seek information from coroners and families, or investigate all of the cases that were reported to it.

The government watchdog warned that although the DWP said it regarded the internal investigations as a way of improving the safety and quality of its services, it had admitted that it has no idea whether lessons from the reviews were ever learned or their recommendations ever implemented.

Joy Love, the mother of Jodey Whiting, a disabled parent with a history of mental illness who took her life three years ago after her benefits were stopped, told the Guardian that without a full public inquiry into benefit-related deaths there was a danger that “nothing will change and these tragedies would keep happening”.

The NAO undertook the investigation last year after the former MP Frank Field – at the time the chair of the work and pensions select committee – wrote to it to complain that the DWP had blocked his requests for data on suicide-related deaths on the grounds that to gather the information would be too expensive.

Field, no longer a serving member of parliament since December’s general election, said: “This report presents a catastrophic situation for vulnerable claimants and their families. What we need now is a full investigation into the DWP’s processes, and for the necessary changes to be made, so that nobody is ever put into this situation again.”

Labour’s shadow disability minister, Marsha De Cordova, also called for an independent inquiry and accused the DWP of a lack of respect for parliamentary scrutiny. She said: “This is heartbreaking, and families who have been affected deserve answers and all the support we can give them.”

A DWP spokesperson acknowledged the NAO’s findings and said that it took them “extremely seriously”. “Suicide is a devastating and complex issue,” the spokesperson said. “We are urgently working to drive forward improvements and learn the lessons from these tragic cases. We will now carefully consider the NAO’s findings as part of our ongoing work.”

The DWP has promised a review aimed at improving its responses to benefit-related deaths, and has plans to set up a Serious Case Panel to investigate the most complex cases. However, this has already run into criticism for failing to involve the families of claimants.

Concern over deaths of vulnerable benefit claimants, often after benefits were withdrawn, leaving them stressed and penniless, has been growing in recent months. Of nine such cases in recent years known to the Guardian, suicide was identified as the formal cause of death.

Last week the Guardian reported the death by starvation in 2018 of Errol Graham, a 57-year-old grandfather with a history of mental illness. Graham died months after his benefits were cut off after he failed to attend a fit-for-work test. A candlelit vigil and protest over Graham’s death was held in Nottingham on Friday night.

Alison Turner, Graham’s daughter-in-law, added her voice to calls for a public inquiry saying she had little confidence in the DWP’s ability to change its ways. “They are going to have to work hard to earn our trust, and they have to start by being open and honest with people.”

The NAO’s report published on Friday reveals that 69 internal process reviews (IPRs) into claimant suicides where “alleged department activity” may have contributed to the claimant’s death have been completed by the DWP since 2014-15. The figure does not include benefit-related deaths which were not a result of suicide.

Of the 69, almost a third were carried out in a seven-month period between April and November last year, reflecting a more proactive approach from the department under former work and pensions secretary Amber Rudd. “This is partly a result of investigating more cases where information received from the media was the trigger [for the investigation],” the NAO said.

However, because there was no clear route for such cases to be communicated to the department, and because there was confusion internally as to whether a reported case should be investigated, it is likely incidents that should have been investigated will have slipped under the radar, the NAO says.

“It is highly unlikely that the 69 cases the department has investigated represents the number of cases it could have investigated in the past six years,” it concludes.

The IPRs do not come to a judgment as to whether benefits-related issues were the cause of the suicide, but instead scrutinise departmental processes and “identify recommendations for change to the customer journey”, which are then in theory passed on to frontline teams.

However, the NAO notes there was no systematic tracking or monitoring of the lessons that emerge from the IPRs. “As a result, the department does not know whether the suggested improvements are implemented.”

The DWP does not seek to identify wider trends that emerge from across the IPR reports, the NAO notes, and the reports themselves are restricted internally. This means “systemic issues which might be brought to light through these reviews could be missed”.

Michael Paul from Disability Rights UK said: “We hear repeatedly about improvements to processes. But the stark reality appears very different. It’s yet more evidence that the welfare benefits system is unfit for purpose. The system should be supporting disabled people who need it, not putting them at risk”