The number of ambulances called out to police custody suites in London more than doubled, from 2,374 to 5,018, in the past four years, a rise that critics say exposes the shortage of nurses to assess and treat detainees.

The call-outs are a drain on resources, with two police officers required to escort suspects to hospital A&E departments, where they can be required for hours.

The Metropolitan police has found it difficult to recruit sufficient nurses for its custody suites. A recent briefing document states that, in spite of their repeated efforts, the “Met’s inability to recruit and retain sufficient numbers of custody nurses was a major concern”, with analysis showing that in March 2016 there was a 70% vacancy rate.

There have been 49 deaths in or following police custody in the Metropolitan police district since April 2002, according to a report from the London assembly’s police and crime committee released in 2014, with the treatment of mental health detainees an area of particular focus.

The Met has stated that the health and wellbeing of individuals in custody was a “key priority”.

Caroline Pidgeon, the leader of the Liberal Democratic London assembly group, and a member of the Metropolitan Police Authority, who has been campaigning for more medical staff to assess and treat detainees in custody, said: “The huge increase in ambulances called out to London police stations in just four years is a clear demonstration that the Met is failing to provide adequate healthcare provision within its custody suites.

“It is an alarming situation when ambulances are routinely being called to police stations, putting additional pressures on our stretched ambulance services.

“This also means the Met are tying up the time of their own police officers, who need to escort detainees to hospital, often having to wait many hours with them at busy A&E departments.”

She added: “The Met needs to get its own house in order and provide comprehensive medical care in its custody suites. It is wrong that another blue light service is having to pick up the pieces.”

The Metropolitan police have 29 custody suites open 24 hours every day of the week, with the number of people taken into custody by the force forecast to be less than 200,000, which represents a significant drop compared with 250,000 in 2014, as the force increasingly moves to the use of community resolution approaches as alternatives to custody.

The Met has tried numerous ways to recruit more nurses, including providing salary and other working incentives, and have stated that the current healthcare arrangements have not increased the risk of death in police custody.

In a statement, the Metropolitan police said: “We take our duty of care extremely seriously and recognise that we are often dealing with some of the most vulnerable individuals within our community.”

Tomorrow national changes to bail conditions will mean that suspects can be “released under investigation” instead of “on bail” before facing possible charges. The introduction of the Policing and Crime Act 2017 will change the way officers across England and Wales deal with pre-charge bail, meaning that police will now presume individuals will be released without bail and instead are “under investigation”, unless specific criteria are met.