Vulnerable people in crisis may "fall through the cracks" if police go ahead with plans to stop responding to mental health calls, a watchdog has warned.
Later this year, Norfolk Constabulary is planning to phase in a scheme called Right Care Right Person - which will see it only respond to mental health calls if a crime has been committed or a life is at risk.
The controversial approach is currently used by police in Humberside and is being adopted by The Met Police in London.
But Healthwatch Norfolk, an organisation which represents patients in the county, has warned the plan could backfire and put vulnerable people at risk.
Alex Stewart, its chief executive, has written an open letter highlighting his concerns about the plans.
He said: "While we agree that a significant amount of police time available for investigating and preventing crime is taken up by officers attending to people in mental health crisis we question how those people will be guaranteed to be seen by the relevant mental health professional or receive appropriate treatment.
"The stark reality is that people in crisis will fall through the cracks."
Mr Stewart's letter was sent to Tracey Bleakley, chief executive of NHS Norfolk and Waveney, and Giles Orpen-Smellie, Norfolk's police and crime commissioner.
In it, he asked for further clarity on how the scheme will be phased in and what assurances can be offered to anybody who may need care in the future.
In response, Ms Bleakley said: "We are very mindful of the need to carefully plan how we implement this, so that we meet what is expected of us nationally, but that we do so in a way that will work in our local context.
"The aim of Right Care, Right Person, is to not stop police from continuing to perform their key role of keeping people safe.
"Where there is a real and immediate risk to life or serious harm - whether that be a person seeking to harm themselves or others - officers will respond swiftly as they currently do."
Ms Bleakley said efforts were already being made to improve crisis care, including establishing a 24/7 helpline, opening five wellbeing hubs and introducing two mental health response cars.
She added: "We have more to do, in particular, to embed the changes we have made and to join up crisis care, all of which we are working on at the moment.
"We haven't seen the full benefit of the changes we have made yet."
Mr Orpen-Smellie said: "I received a letter from Healthwatch Norfolk on Thursday and have responded to Alex Stewart informing him that a programme of briefings with stakeholders has just begun.
"I have invited Healtwach Norfolk to a briefing, at the earliest possible opportunity, on what is a sensitive issue for all our stakeholders."
Mr Stewart added: "We recognise that addressing these issues requires support from a number of organisations, funding, ongoing training and evaluation which is a lot of work.
"Equally, our role is to help ensure the people of Norfolk are getting the support they need when they need it and it would be wrong of us not to highlight our concerns at this stage.
"We are very aware of the work that is going on to improve mental health care in Norfolk and we would be worried if this very sudden decision is a factor in undoing some of the progress that has been made in recent months."
The Right Care Right Person scheme was first introduced in Humberside three years ago, but is gradually being rolled out in other areas.
Norfolk Constabulary said it would be taking a phased approach to introducing it locally later this year.
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