A police force has taken the unprecedented step of admitting breaking health and safety law over the death of a church caretaker who collapsed in custody after a heavy webbing belt was held across his face.

The office of the Devon and Cornwall chief constable pleaded guilty to breaches under the Health and Safety at Work Act on Friday – six years after the death of Thomas Orchard, who had mental health problems.

It is the first time a British police force has admitted to an offence in such circumstances and it is being seen as a landmark case by campaigners, who believe police forces and officers almost always escape justice after preventable deaths in custody.

Devon and Cornwall police admitted breaching the act on the basis that there were failings over issues such as how officers were trained in the use of the US-made emergency response belt (ERB).

However, the force does not accept that the belt directly caused Orchard’s death, and a judge will rule whether the ERB was a significant factor in the fatality after considering evidence at a further hearing in April. The judge will sentence the force in May.

It is understood that if, ultimately, a judge rules the ERB caused Orchard’s death, the Devon and Cornwall chief constable, Shaun Sawyer, will consider his position. He was deputy chief constable when Orchard died.

Orchard’s family is not satisfied with the force’s admission and believe it should have been charged with corporate manslaughter.

Speaking outside Bristol crown court, Orchard’s father, Ken, said: “For over six years Devon and Cornwall police have consistently refused to accept any responsibility for Thomas’s needless and avoidable death. We welcome the guilty plea and the fact they are finally admitting to some failings.

“However having watched and listened to much evidence over the years we’re shocked and horrified that they are still denying that those failings in any way contributed to Thomas’s death.

“We will continue to fight for justice for Thomas and are committed to doing all we can to reduce the shameful frequency with which people with mental health difficulties die in police custody.”

Orchard’s family have described him as a sensitive, free-spirited man who began to have mental health problems after taking recreational drugs as a teenager, and argue he was badly let down by the police, claiming officers should have treated his case as a medical emergency rather than a criminal incident.

Orchard, who had paranoid schizophrenia, was arrested after a disturbance in Exeter in October 2012 and taken to the city’s Heavitree Road police station.

At the trial of three officers accused of his manslaughter, prosecutors said the ERB was wrapped tightly around his face when he was carried in a prone position and then placed face down in a cell, restricting his ability to breathe. He went into cardiac arrest and died a week later. He was 32 years old.

The officers insisted the ERB was used to stop Orchard biting or spitting at them and claimed it was an approved piece of equipment supplied by Devon and Cornwall police for that purpose. They were cleared of manslaughter.

Lawyers for the family believe this is only the third time a force has been prosecuted under the Health and Safety at Work Act 1974 in relation to a civilian’s death.

The other two were the cases of Jean Charles de Menezes and Anthony Grainger, who were both shot dead by police. The latter case was subsequently dropped. The Met was fined £175,000 and ordered to pay £385,000 costs over the De Menezes case. However, the Orchard case is the first involving restraint in custody.

Play Video 2:28 Devon and Cornwall police chief: 'It is only right to plead guilty' – video

The office of the chief constable – the corporate body for the Devon and Cornwall force – admitted breaches under section 3 and 33 of the act, which states that organisations must not expose non-employees to risk. Sawyer was present in court as the holder of the office but was not required to sit in the dock and did not speak.

It is understood the force’s plea is based on issues such as how it procured the ERB, how officers were trained and risk assessments around its use.

In April there will be a “Newton hearing” before Judge Julian Lambert to consider the prosecution’s claim that the ERB caused Orchard’s death by restricting his breathing.

That hearing may also look into the matter of who – if anyone – in the force said the ERB could be used around the face as a spit hood and forensic pathologists are to give their assessments of Orchard’s condition at the time of his arrest.

Devon and Cornwall police still use ERBs as a limb restraint, but do not use them around the face as spit hoods.

In a statement outside court, Sawyer offered his “deepest regret” to Orchard’s family and friends but did not apologise.

He said: “From 2002 until 2012 Devon and Cornwall police researched, procured and trained members of the organisation in the use of an emergency restraint belt. That piece of equipment was used during the restraint of Mr Thomas Orchard. Following his restraint and detention, Thomas became unconscious and a week later died.

“In respect of the use of the emergency restraint belt, it is my belief that standards expected by Devon and Cornwall police were not met between 2002 and 2012 and did not meet the legal threshold of the Health and Safety at Work Act 1974.”

Deborah Coles, the director of the charity Inquest, said: “This historic guilty plea is long-awaited acknowledgement by Devon and Cornwall police of the criminally unsafe equipment used in Thomas’s brutal restraint. It should not have taken his needless death to expose the dangerous and unregulated use of this restraint equipment.

“This plea comes against a backdrop of escalating use of force by police, a rise in restraint-related deaths and deaths of those in mental health crisis. Corporate responsibility for dangerous culture and practice within policing is rarely the focus of scrutiny and accountability. This has got to change.”