Growing numbers of people being treated in mental health units are harming themselves and trying to take their own lives, new NHS figures suggest.

The number of such incidents at 29 of England's 52 NHS mental health trusts rose from 14,815 in 2010 to 23,053 last year, an increase of 56% over four years.

The average number per trust rose from 511 to 795 last year over the same period.

Labour, which obtained the figures under freedom of information laws, linked the increase to cuts in the number of doctors and nurses working in mental health units and their budgets.

"Mental health services have been squeezed year on year, the number of specialist doctors and nurses has dropped and there aren't enough beds to meet demand. The pressure this is putting on mental health wards is intolerable", said Luciana Berger, the shadow public health minister, who obtained the figures.

"It is unacceptable that people in touch with mental health services may not be getting the support they need. They are some of the most vulnerable patients in our NHS", she added. That some mental health wards were running way above their recommended maximum capacity of 85%, sometimes reaching as much as 138%, was also a factor explaining the rise, Berger added.

She also highlighted the fact that chronic shortages of beds in some mental health units were leading to patients who are undergoing a crisis having to travel sometimes hundreds of miles from their home to receive treatment.

The biggest increases were at Northumberland, Tyne and Wear NHS Foundation Trust, where such incidents rose from 2,008 to 3,746, and at Nottinghamshire Healthcare NHS Trust, where they rose from 2,071 to 3,935. The other 21 of the 50 trusts that Labour approached did not respond, so the true figures for England as a whole might be much higher.

The latest NHS statistics show that at the end of May some 963,769 people in England were in contact with secondary mental health services, of whom 23,646 (2.5%) were inpatients in a psychiatric hospital. Of those, 16,352 had been sectioned under the Mental Health Act 1983, and 11,965 (73.2%) of those were detained in hospital, and most of the rest – 4,228 (25.9%) – were subject to a community treatment order.

Mark Winstanley, chief executive of the charity Rethink Mental Illness, said the findings were clear evidence of the marked deterioration of mental health services, because of disproportionate funding cuts imposed over the past two years. "The hollowing-out of early intervention and cutbacks in community support services are leading to people ending up in inpatient care after becoming much more seriously ill because they haven't been helped thoroughly or quickly enough in the first place", he added.

However, Professor Sir Simon Wessely, the new president of the Royal College of Psychiatrists, did not believe the conclusions put forward by Labour, and said that increases were seen because of better reporting since Robert Francis's report into the Mid Staffs hospital scandal in early 2013.

The number of suicides among mental health inpatients, which has fallen every year since 2004, was a better measure and did not fit with the picture revealed by the 29 trusts' data, Wessely explained.

"The concerns about [lack of] funding are genuine and the concerns about staffing are genuine, but I just don't think we can link that to deliberate self-harm", he said. The NHS figures given to Berger were "implausible", he added.