Police cells were used to detain people suffering mental health problems on more than 4,500 occasions in the last year, new figures reveal.
There were a total of 4,537 instances of police custody being used to accommodate individuals held under section 136 of the Mental Health Act in 2014/15.
Data collected from police forces in England and Wales showed that this was a fall of almost a third (32%) compared to the previous year, when the practice was used 6,667 times.
The detention in police cells of those experiencing mental health crises who are not suspected of any crime has been controversial.
Last month Home Secretary Theresa May announced up to £15 million of new funds to provide health-based alternatives.
Section 136 provides police with the power to remove someone considered to have a mental disorder to a "place of safety" and detain them without their consent. They can be held for a maximum of 72 hours.
The new figures, obtained by the National Police Chief's Council (NPCC), showed that the number of instances of children under 18 being taken to police custody under the legislation also fell, dropping from 256 in 2013/14 to 161 in 2014/15.
In total section 136 was used on 23,128 occasions by police officers in 2014/15, a fall of more than a tenth compared to the previous year.
Commander Christine Jones, NPCC lead for mental health and policing,said:"People experiencing a mental health crisis are not criminals. Nor are police officers and staff medical professionals.
"Those who are urgently unwell, whether physically or mentally, should be taken to a health-based setting to receive the right care and support.
"It's unacceptable for anyone, of any age, to end up in a police cell because the appropriate mental health services are not available.
"The data published today shows that the message is getting through. By working more effectively with local partners, and improving understanding across the service, we are helping to reduce the number of vulnerable people being detained in police custody under the Mental Health Act.
"However, there is much more to be done and we will need support from other agencies if we are to achieve our end goal."
The Home Office said the figures show "encouraging progress" but "in some areas there is still a long way to go to improve outcomes for people with mental health needs".
A spokesman added: "The Home Secretary is clear that the right place for a person suffering a mental health crisis is a bed, not a police cell, and the right people to look after them are medically trained professionals, not police officers."
The data showed wide regional variations in the use of police cells, from no instances in some areas to hundreds in others.
Sussex had the highest total number of instances, with 765 in 2014/15.
A spokesman for the force said there was a "downward trend" in the practice in recent years.
He added: "Once police custody ceases to be a routine place of safety then all people detained by police under the Mental Health Act, unless exceptional circumstances prevail, will be taken to other places of safety for their assessment."