Ministers should end the "scandal" of vulnerable children and young people suffering a mental health crisis being assessed in a police cell because of a nationwide shortage of proper psychiatric facilities, an influential MP has demanded.

Dr Sarah Wollaston, the chair of the Commons health select committee, said it was "wholly unacceptable" for under-18s who are picked up by the police because they are having a breakdown to be taken into cells rather than to a specialist medical unit.

"It would be unthinkable for someone who had a broken leg, for whom there was no place to assess them in casualty, to be taken to a police cell. It should be unthinkable for someone who's having an acute mental health crisis to be seen in a police cell. That's inexcusable, but it's happening," Wollaston told the Guardian. "That's wholly unacceptable for an adult, much less for a child."

Wollaston, who was a GP for 24 years before becoming Conservative MP for Totnes in 2010, added: "We won't have true parity of esteem [between mental and physical health in the NHS] unless we end the scandal of section 136 assessments".

Anyone detained under section 136 of the Mental Health Act 1983, which often happens if someone appears mentally disturbed in a public place, should be assessed in a "place of safety". That is meant to be a mental health unit, but a lack of them in many parts of England means one in three assessments takes place in a cell in a police station.

Figures from the Care Quality Commission, the NHS care regulator in England, show that 21,814 assessments of adults or children took place in 2012-13 under section 136, of which 7,761 involved the use of a police cell. According to the Association of Chief Police Officers, a disproportionate 45% of under-18s detained under section 136 that year were assessed in police cells.

Victoria Bleazard, director of communications and campaigns at the charity Rethink Mental Illness, said: "It's disgraceful that so many people, especially children and young people, are being held in police cells under section 136. If you're experiencing a mental health crisis, you should be brought to a 'place of safety' in a health facility. A police cell should only be used as an absolute last resort."

However, she added, there are only 161 places of safety in England, many of which can only take one person. "That means that there simply aren't enough facilities available to meet the needs of people in crisis. Even worse, more than a third (35%) of those facilities do not accept young people under the age of 16, which is why children in crisis are being taken to police cells," said Bleazard.

The situation is compounded by the fact that the strict admission criteria enforced by many places of safety mean some do not accept people who show signs of drug or alcohol abuse. "That's a big problem, because a high proportion of people going through a mental health crisis will also have related drug and alcohol issues," she said.

Wollaston accused the NHS of treating mental health as "a Cinderella service" and seriously underfunding it. NHS bosses had failed so conspicuously to deliver the coalition's landmark pledge of "parity of esteem" between mental and physical health that the health secretary, Jeremy Hunt, may be forced to intervene, she said.

Wollaston said the NHS had given mental healthcare low importance for decades.

"Mental health has always been a Cinderella service within the NHS, underfunded and given nowhere near enough priority. There's never been a time when mental health services have been funded equitably with physical health and received a fair share of the cake, especially child and adolescent mental health services," she said.

The last Labour government's decision not to set maximum waiting times for patients to access mental health treatment, despite introducing them for key physical health services such as A&E units and planned operations, was an example of the inferior status psychological illness has received from Whitehall and the NHS, she added.

The coalition has pledged to rectify that imbalance by bringing in waiting times for mental health treatment from next April, though practicalities are still being discussed.

Mental health conditions, such as anxiety, depression and schizophrenia, make up 23% of all the conditions dealt with by the NHS. But the service in England spends just 13% of its annual £110bn budget on diagnosing and treating mental ill-health.

Fewer than a third of people with common mental health problems get any treatment at all, the new president of the Royal College of Psychiatrists, Prof Sir Simon Wessely, told the Guardian last week.

Calling for more money for mental health, Wollaston said: "It's got a very small share of the funding, given the morbidity associated with mental health problems. We all know that the big challenge for mental health, especially for child and adolescent mental health services, is the funding issue. NHS England and Monitor [the NHS's financial regulator] need to set out very clearly how the financial levers within the NHS are going to be changed … because the problem at the moment [is] the way that funding works across the NHS has always disadvantaged mental health treatments compared to physical health treatments."

Higher spending would ultimately save the NHS money by helping keep people with mental health conditions healthier for longer and reduce early death, Wollaston believes.

Freedom of information responses obtained last month by Labour showed that some of the 211 GP-led local clinical commissioning groups across England spend as little as 6.6% of their budget on mental health. Those figures prompted Luciana Berger, the shadow public health minister, to complain that patients were facing an "alarming postcode lottery" in access to treatment as a result of the huge variation in spending.

Bleazard endorsed Wollaston's criticisms. "She is absolutely right that people needing mental health support are still receiving a second-class service, despite the government's promise to value mental health equally to physical health in the NHS. It's no surprise then that there is a huge gulf between the demand for services and the support that is actually available to people in need. Our supporters say the situation is getting even worse because of cuts."

Many people experiencing a mental health crisis are waiting for years for treatment, for example under the early intervention in psychosis scheme, while others miss out altogether or receive substandard care because services near them cannot cope with demand, Bleazard added.

Wollaston was sharply critical of NHS England for not having translated ministers' pledge of parity of esteem between physical and psychological ill-health into action. "It's positive that the [Department of Health's] mandate [to NHS England] says that there should be parity of esteem. But that mandate must be put in place. Delivering parity of esteem is the intention, but it's not happening at the moment. What's frustrating for people who suffer from mental illness and the people who care for them is that the pace of change has been very slow," she added.

NHS England and Monitor should ensure both types of services were treated equally within a year or the health secretary will have to step in to ensure the gap between them is finally closed, she said.

Wollaston cited last year's decision by NHS England and Monitor to cut the amount of money hospitals receive for treating psychological ill-health by 20% more than it reduced fees for physical health care as evidence of a deeply ingrained bias against mental health.

She accused the NHS of endangering the health of people with severe mental illness, such as schizophrenia.

"Those people die between 15 and 20 years earlier than everyone else. That's very often because their physical health is being ignored, so they are at greater risk of dying of a stroke, for example."

Dr Martin McShane, NHS England's director for people with long-term conditions, said the organisation was working hard, and using its £96bn annual budget, to improve care and services for people with mental ill-health. But most decisions about what services are available are taken locally by England's 211 local NHS GP-led clinical commissioning groups, McShane said.

It is supporting clinical commissioning groups to deliver high quality care and parity of esteem, and the latter is "a big priority" for NHS England, he added. Access to psychological therapy is increasing, care for those undergoing a mental health crisis is being improved and it is working with Monitor, the NHS's economic regulator, to devise new payment systems to incentivise CCGs to provide good quality care to the mentally unwell, McShane added.

It is also making 50 more beds available nationwide to boost child and adolescent mental health services and hiring 10 to 20 new case managers after sustained criticism of the service recently, he said.

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