A woman with a history of stress-related depression killed herself because her benefits were cut after a work assessment, a health watchdog has found.

The Mental Welfare Commission for Scotland said the woman in her early 50s took her own life less than a month after an Atos assessor gave her zero points in a work capability assessment and her weekly benefits were docked by nearly 30%.

The MWC said it could find no other reason why the woman, named only as Miss DE, would kill herself at her home on New Year's Eve 2011.

She had no history of suicidal behaviour, was hoping to return to work and was about to get married.

After an exhaustive investigation, including interviews with all the mental health professionals involved in her treatment, her GP, friends and local welfare rights team and the Atos and Department of Work and Pensions (DWP) staff involved in her case, the commission concluded the assessment was to blame.

Dr Donald Lyons, chief executive of the MWC, said: "This lady had a lot to look forward to. She was getting married. She was being treated. She was undertaking voluntary work. She had a good social network.

"There wasn't anything else which we could identify that would lead us to believe that there was any other factor in her life that resulted in her decision to end her life."

In its stinging report on Miss DE's case published on Wednesday, the commission called for the DWP to overhaul its rigid work capability assessments procedures by making them more responsive to people with mental health histories.

The overhaul should include making it routine that medical reports are collected in mental health assessments, that at least two sources of information are used and that the person's GP and psychiatrist should be told of the "potentially challenging situation" they faced.

After disclosing that a senior NHS official had alerted the MWC to Miss DE's case, the commission said it had investigated because it knew of "numerous reviews, research and widespread public debate on this subject".

"One of the reasons we undertook this investigation is because the issues identified may affect many people in similar circumstances," it said, adding: "A number of clinicians had expressed concern about the impact on patients of this process and reassessment."

The MWC's survey of Scottish psychiatrists found that 13% reported that at least one of their patients had attempted suicide after going through a work capability assessment, and 75% of them said neither the DWP nor Atos had asked them to take part in the assessments or provide medical evidence about their patients.

In Miss DE's case, she was given an hour-long assessment by an Atos-employed doctor for the DWP, but was not asked to complete a self-assessment questionnaire. No evidence was sought from her psychiatrist or her doctor.

The Atos assessor concluded that Miss DE presented "no evidence that she has a significant disability of mental health function". She was told by letter on 9 December 2011 that she had scored zero points, after two unsuccessful attempts to call her to explain the decision.

A welfare rights officer then explained to Miss DE she would therefore lose her incapacity benefit of £94.25 a week, and be put on £67.50 a week jobseeker's allowance instead. The welfare rights worker said Miss DE became very upset and worried about paying her mortgage.

The DWP said it and its advisers had already examined many of the issues raised by the MWC, as had an independent tribunal and separate reviews, and they had decided its systems were largely robust and correct.

It added that it would work with Atos and stakeholder groups to improve the way it informed claimants about the process and its questionnaires, and would ensure that assessors made clear they did not have access to claimants' medical notes.

"DWP remains committed to keeping their processes for collecting further evidence under constant review – and will improve these processes where possible," the department told the MWC.

"It remains important to retain a balance between the added value of further evidence in any claim for Employment and Support Allowance and the demands on the time of GPs and other healthcare professionals."

• This article was amended on 27 March 2014 to clarify that the decision to cut benefits was not made by the assessor.