From September, officers will only attend mental health 999 calls where there is an "immediate threat to life". The Met argues the move will free up officer resources after a significant rise in the number of mental health incidents being dealt with by its officers in the past five years.

Some police chiefs believe this is down to the service increasingly being seen as the first resort for people in a crisis, as well as a lack of capacity in the NHS and social services to deal with growing mental health demands.

Sarah Hughes, chief executive of Mind, has warned there is not enough capacity in other public services to replace the work police officers currently do.

Zoe Billingham, who was previously Her Majesty's Inspector of Constabulary and Fire and Rescue, said the proposals could create a "vacuum". Speaking to BBC Radio 4's Today programme, she warned there is "simply no other agency to call" other than the police for people in crisis.

Ken Marsh, chairman of the Met Police Federation - which represent officers up to the rank of inspector - broadly welcomed the announcement. He said police officers can be required to wait in hospital for up to 12 hours with people suffering from mental health problems or dementia.