A poet and teacher with mental health issues died after he was restrained in a prone position at a police station in west Wales, an inquest jury has found.

Meirion James, from Pembrokeshire, was restrained by police officers and civilian detention officers, and also sprayed with Pava spray – a type of incapacitant.

A jury at Haverfordwest said it was more likely than not that the manner of James’s restraint caused or contributed to his death.

The panel also judged that the failure to carry out a mandatory mental health assessment the day before 53-year-old James died also contributed.

Speaking outside court, James’s sister, Diana Vaughan-Thomas, said: “Meirion was my much-loved brother and a wonderful son and uncle.

“It’s been a painful wait for this inquest, and a tough few weeks hearing the evidence. He shouldn’t have died. I hope that lessons will be learned.”

Clare Richardson, solicitor for the family, said: “The jury in this case heard shocking evidence, both about the missed opportunities as Meirion’s mental health deteriorated in the days and hours before his death and about the fatal restraint. This inquest jury has finally confirmed the link between the dangerous restraint and the death.”

James had suffered bipolar disorder since his 20s but he had been prescribed lithium for many years and lived a quiet life with his elderly mother in rural Wales.

In the autumn of 2014, James’s treatment changed. He was taken off lithium because of concerns that it was harming his kidneys, and his mental health worsened.

On 30 January 2015, James was involved in a minor car accident and the police were called. It was clear he was unwell and he was detained under section 136 of the Mental Health Act, which allows police to remove a person from a public area.

James ended up at Bronglais hospital in Aberystwyth via a police station. The use of section 136 means he should have undergone a mental health assessment, but this did not happen and he was discharged.

After returning home, James became aggressive and assaulted his mother. He told her she had the devil in her and the mafia were after him. James himself called the police and he was arrested.

He was put into a cell at Haverfordwest police station. He spoke in a child-like voice and pulled out clumps of his hair. He stuffed a blanket and his own trousers into the cell toilet and asked to see a doctor. He was assessed by a medic who concluded he was fit to be detained.

When officers went to James’s cell at about 11am on 31 January, he burst out of the cell and into the corridor.

Officers restrained him. It is not clear how but he ended up on his front, though two factors – his obesity and mental vulnerability – made positional asphyxiation a risk, the inquest heard.

James became unresponsive, and despite efforts to resuscitate him at the police station and at Withybush general hospital in Haverfordwest, he was pronounced dead at 11.30am.

Asked if it was more likely than not that the manner and/or position of restraint caused or more than minimally contributed to his death, the jury answered: “Yes.” It added: “Due to excessive length of time in the prone position and Mr James’s size, this resulted in his death.”

The jury was also asked: “Is it more likely than not that the failure to carry out a mental health assessment caused or more than minimally contributed to Meirion’s death?” It replied: “Yes.”

Deborah Coles, the director of the charity Inquest, said: “There were many missed opportunities to safeguard the life of a man in mental health crisis. All the police officers knew about the risks of restraint and positional asphyxia and yet they continued to restrain Meirion in the prone position for an excessive length of time, resulting in his death.

“This case calls into question either the quality of the training or the officers’ adherence to it. That a man can die in this way despite repeated recommendations arising from previous deaths is utterly shameful.”

The Independent Office for Police Conduct looked at the case and concluded that two officers had a case to answer for misconduct. They were given “management action” by the force.

Vicki Evans, assistant chief constable of Dyfed-Powys police, said: “Mr James died in tragic circumstances and I cannot begin to comprehend the distress and grief this has caused for his family.

“Ensuring the public are safe is a priority for our officers and staff and we deeply regret it when anyone comes to harm during or following police contact.

“Immediately after the incident, we referred the case to the Independent Police Complaints Commission, now the IOPC. We fully cooperated with their investigation and their recommendations have been actioned.

“Improvements have been made in the four years since Mr James’s death, both in Dyfed-Powys police and nationally, in the way that police support people who are suffering from a mental health illness.”