There has been a 50 per cent jump in the number of "unexpected" deaths among mental health patients in the last three years, according to a report by Panorama.
Unexpected deaths, a term that covers deaths caused by suicide, neglect and misadventure, are said to have increased from 2,067 in 2012-2013 to 3,160 in 2015-16 among people suffering from depression, anxiety, schizophrenia and bipolar disorder.
The findings, based on reports from 33 of the 57 mental health trusts in England, all of which were sent Freedom of Information requests, have raised concerns that cutting the UK mental health budget has affected quality of care, says the BBC.
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According to think-tank Health Foundation, mental health trusts have had their budgets cut by £150m over the past four years, compared with a rise in national spending on general health of £8bn. Last year, The Guardian reported that one in ten specialist mental health nurses had been cut over the past five years.
However, the Department of Health says the increase is due to a change in the way these deaths are recorded and investigated.
"The NHS is very deliberately improving the way such events are recorded and investigated following past failings," said a spokesman. "From April, all NHS trusts will be required to publish both numbers of avoidable deaths and how they are improving care.
"We also dispute the funding figures used in this programme."
Mental health campaigner and Liberal Democrat MP Norman Lamb called for an "urgent inquiry" into the report.
"The government and NHS England have a responsibility to act on this. It is incumbent on the government to establish an urgent inquiry," he said. "We must get to the bottom of variations in death rates."
Leading mental health charity Mind says that one in four people in the UK will suffer from a mental health problem each year.
Around 280,000 people are treated for schizophrenia by the NHS, while a further 2.4 million are said to have bipolar disorder, says Psychologies.
Panorama's Revealed: Britain's Mental Health Crisis is on BBC1 at 8.30pm today.
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