Devon and Cornwall Police has pleaded guilty to health and safety breaches following Thomas Orchard's death in custody in 2012.
Mr Orchard, 32, who had paranoid schizophrenia, died in hospital seven days after being arrested and taken to Heavitree Road police station in Exeter.
During his detention, Mr Orchard was restrained and an emergency response belt (ERB) was placed across his face.
He was left in a locked cell and he lay there apparently motionless for 12 minutes before staff re-entered and commenced CPR.
In March 2017, a custody sergeant and two staff members from Devon and Cornwall Police were acquitted of Mr Orchard's manslaughter by gross negligence.
The following year, a prosecutors charged the office of the chief constable of the force under the Health and Safety at Work Act 1974.
The chief constable of Devon and Cornwall Police released a statement following the force's guilty plea at Bristol Crown Court on Friday. No individual person is being prosecuted.
Shaun Sawyer expressed his "deepest regret" to the victim's family.
"In the intervening six years [since Mr Orchard's death], my thoughts have always been with Thomas, his family and friends who have lost a loved one.
"It is only today that I have been able to personally offer my deepest regret to all those individuals."
Mr Sawyer said it was his belief that the use of the belt in that way fell short of health and safety standards and the forces' own standards.
He continued: "In the spirit of candour and out of respect to the courts, the family of Thomas, the public and my workforce, I have decided – as the corporate responsibility of this organisation, that it is only right to plead guilty on behalf of Devon and Cornwall Police to this charge.
"However, legal matters remain outstanding in respect of whether this health and safety breach caused the death of Thomas."
The hearing, which is expected to last for three days, will take place in April next year and has been welcomed by Mr Orchard's family.
Speaking outside court, Ken Orchard, his father, said: "For six years, Devon and Cornwall Police have consistently refused to accept any responsibility for Thomas's needless and avoidable death.
"Having watched and listened to much evidence over the years, we are shocked and horrified that they are still denying that their failings contributed to his death.
"Whilst our family does not relish the prospect of yet another hearing, we are pleased that the defendant's denial that its criminal conduct caused Thomas's death will now be tested in open court.
"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."
The April hearing will not look at whether or not restriction of breathing by the belt was a contributory factor of death.
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It will include evidence of Mr Orchard's restraint, including CCTV footage and witnesses.
Jason Beer QC, representing the office of the chief constable for Devon and Cornwall Police, told the court: "The principal issue between the parties is causation and there is a subsidiary issue of training."