The way a woman was assessed for benefits led to her suicide less than a month later, according to a mental health watchdog.
The woman had a history of depression and was on significant medication, but scored zero points in a Work Capability Assessment (WCA), carried out by Atos.
A Mental Welfare Commission report said it could see no other factor "in her decision to end her life".
The Department for Work and Pensions said correct procedures were followed.
The woman, who is identified only as Miss DE, was in her early 50s and had been out of work for just under two years due to stress-related depression when she was assessed for Employment and Support Allowance (ESA).
ESA replaced incapacity benefit as part of changes to the benefits system, introduced by the UK government in 2007.
Miss DE did not receive a self-assessment questionnaire and no evidence was requested from her psychiatrist or GP.
The doctor who conducted the hour-long assessment for Atos, on behalf of the DWP, concluded that Miss DE showed "no evidence that she has a significant disability of mental health function" and she was notified by letter that she had scored zero points in the assessment on 9 December 2011.
When a welfare rights officer informed Miss DE that this would mean her £94.25 per week incapacity benefit would be reduced to a Jobseekers Allowance of £67.50 per week she became very upset and said she did not know how she was going to pay her mortgage.
She took an overdose on New Year's Eve.
"This lady had a lot to look forward to," said the chief executive of the MWC, Dr Donald Lyons.
"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."
When a DWP representative analysed the process, he told the MWC that the steps taken showed "nothing untoward."
The MWC said a survey of psychiatrists conducted as part of its investigation found that 13% reported that at least one of their patients had attempted suicide as a result of the assessment process.
A total of 75% said they had not been asked by the DWP or Atos to take part in benefit assessments, although the majority said their patients had asked them to provide medical evidence.
About 85% of the psychiatrists said that the benefits assessment had led to patients needing more frequent appointments.
The MWC said there were examples of patients who had stopped receiving ESA despite their doctors being adamant that they were completely unable to work.
A spokesman for the DWP said: "This was a tragic case and our thoughts go out to the family. We conducted an internal investigation which found our procedures of the time were correctly followed and the relevant safeguards implemented.
"This report is narrowly focused using a single case from 2011 to make conclusions about the Work Capability Assessment process without taking into account the significant improvements we have made - and continue to make - for people with mental health conditions.
"We worked with the Mental Welfare Commission throughout their review and formally responded to their recommendations with a commitment to further improve our processes where required. "
The mental health watchdog, however, has said that more change is needed.
Dr Lyons added: "There are some things that haven't changed and we still don't see a commitment from the DWP to changing.
"One of those is the need to have more than one piece of evidence before making a decision like this.
"We think the DWP should pay very careful attention to the lessons in this report. If they do, it will make it less likely that others will be as distressed or - heaven forbid - take their own lives the way that Miss DE did."
The MWC has recommended that medical reports should be routinely obtained for anyone with a mental illness or learning disability and says that it has "major concerns" that the Work Capability Assessment is not sensitive enough to capture the elements of mental illness which mean that a person is unable to work.
Atos, which is in the process of negotiating an early exit from its contract with the DWP, said: "We understand that applying for benefit can be a difficult and emotional time which is why we work very hard to try to make the part of the process we are responsible for as comfortable as possible.
"The Work Capability Assessment was designed by the government as a way of assessing how an individual's disability or illness impacts on their day-to-day life. It is not designed to diagnose or treat a medical condition.
"In line with guidance from DW, so as not to overload the GP community, we will request further medical evidence only where this is likely to mean that a person will be eligible for benefit without the need for a face-to-face assessment. We do this in about a third of all cases."