Unexpected deaths among mental health patients under NHS care have soared by more than a fifth over the last three years, according to new figures.
The data, obtained by the Liberal Democrats under Freedom of Information laws, also shows the number of mental health patients attempting or committing suicide has risen by 26% since 2012-13 to 751 last year.
Overall, the number of serious incidents - cases requiring an investigation - climbed by 34% to 8,139 in 2014-15.
Liberal Democrat MP Norman Lamb, a former mental health minister under the coalition government, said some services were "struggling to cope" and had been left "threadbare" by underfunding.
He told the Guardian: "Significant numbers of unexpected deaths at the Mid Staffs NHS trust caused an outcry and these figures should cause the same because they show a dramatic increase in the number of people losing their lives.
"NHS England and the Government should set up an investigation into the cause of this as these figures involve tragedies for families around the country and the human impact is intense."
The figures, from NHS England and covering 58 mental health trusts, show the number of serious incidents has risen year-on-year from 6,074 in 2012-13 and 7,345 in 2013-14.
There were 1,713 unexpected deaths of in-patients or those cared for at home during 2014-15, up 21% from 1,412 in 2012-13.
It comes amid mounting pressure on the health service to improve the way patient deaths are investigated and reported.
Last month Health Secretary Jeremy Hunt ordered a review into NHS trusts following a report into Southern Health, which showed the trust had failed to probe the deaths of hundreds of people since 2011.
Labour mental health spokeswoman Luciana Berger said: "This drastic increase in the number of people losing their lives in NHS care is utterly appalling and tragic for all families involved. It must act as a serious wake-up call for ministers.
"At a time of rising demand, the Government has presided over service cuts and staff shortages, with devastating consequences."