Confidential paperwork about mental health patients - including personal details, medical records and care plans - were found ‘blowing around’ a city centre street.
An investigation has been launched by Sheffield Council into how the papers were lost.
Today a support group working with people with mental health problems condemned the incident as ‘terrible’.
Sheffield Mental Health Action Group urged the council to take disciplinary action against whoever is responsible, and ensure there is greater control of how such sensitive paperwork is carried.
The bundle of documents, which included full names, dates of birth and telephone numbers, was handed to The Star by a concerned passer-by who discovered it in the street.
The person, who wished to remain anonymous, said: “The documents were found blowing around Bridge Street. It is a good thing they did not fall into the wrong hands.”
The documents relate to three separate patients.
In one case, the paperwork relates to a decision to section a woman with a history of absconding from previous addresses, who needed ‘urgent deprivation of liberty’ to prevent harm to herself.
She is a patient at a small care home in the west of the city.
Papers from the second case included a detailed log of a man’s behaviour over several days, along with medical papers relating to an injury he is believed to have suffered and information about treatment received.
Neither of the two care homes involved would comment about the loss of the paperwork.
The third case was of a patient not in a care home, who had been put under a placement with a carer.
The Star has returned the papers to Sheffield Council, which said it was ‘investigating’.
Eddie Sherwood, Sheffield Council’s director of care and support, said: “This is an extremely serious breach of confidential information.
“We are grateful to the member of the public who found the papers for acting responsibly, and to The Star for returning them to us.
“As a large organisation which deals with the public, we have strict procedures in place to stop this from happening.
“Unfortunately this appears not to have happened here and we have launched an immediate investigation.
“People need to have confidence that we will keep their personal details safe, and we will get to the bottom of how this happened.”
Scott Milnes, treasurer at Sheffield Mental Health Action Group based at Castle Market, which supports hundreds of city residents with mental health problems, said: “It’s terrible that this has happened.
“Members of the public shouldn’t be able to pick up really personal paperwork like that in the street. It’s shocking.”
Mr Milnes added: “A big issue for people receiving treatment for mental health issues is paranoia about how their personal details will be handled - and something like this could make them reluctant to seek help or co-operate with health workers.
“Knowing their personal details have been treated in such a way could destroy somebody - people are very private about such information.
“The council needs to change its procedures to ensure a situation like this does not happen again.
“I would also push for disciplinary action against whoever was responsible for this carelessness.”