The number of nurses taking time off due to stress has soared as the NHS has struggled to cope with rising demand for care. Responses to freedom of information requests submitted to NHS organisations by the Observer show that both the number of nurses on stress-related leave and the amount of time taken off are up significantly in the past three years in London, Scotland and Wales.

The figures have prompted claims by health unions that the NHS’s 400,000 nurses are being stretched to breaking point as a result of having to work more demanding and longer shifts at a time when understaffing and the increasing complexity of illnesses add to pressures.

In London, almost 1,500 nurses at 31 NHS trusts took time off because of stress during 2014, up 27% on the 1,179 who did so in 2012. That meant that one in every 29 nurses were off ill with stress. The 1,497 nurses took an average of 38 days off for stress. Some may have had anxiety or depression rather than stress, as the NHS includes those conditions in its definition of stress-related leave. The number of nurses’ working days lost to stress at the 28 acute trusts and three mental health trusts rose from 38,654 in 2012 to 57,156 last year – a rise of 48%.

Writing on observer.co.uk, former Labour cabinet minister Tessa Jowell, who is standing down as MP for Dulwich and West Norwood and intends to stand for mayor of London, says the capital’s hospitals have returned to the sort of crisis levels she saw when she entered parliament 23 years ago. Jowell says many doctors and nurses are leaving or taking time off due to stress because the NHS is failing to provide necessary 24-hour services, as well as care in the community and in people’s homes. “Last week, King’s College hospital in south London had the equivalent of a whole ward full of elderly people ready to go home, another full with people who need never have been admitted,” she writes.

“Doctors and nurses enter their professions motivated by a sacred vocation to care and to make people better. The frustration expressed by so many through stress or by leaving the health service is because they are being denied the ability to exercise their professional calling.”

The seven health boards in Wales have seen the number of nurses off with stress rise 17% from 2,188 in 2012 to 2,563 last year. Those who become so stressed they could no longer work were off sick for an average of 51 days. Scotland also saw stress-related sick days among nurses rise, by 34% from 116,735 in 2012 to 156,880 last year.

The Royal College of Nursing, which represents about 300,000 NHS nurses, said they were “being driven to breaking point”. Kim Sunley, its senior employment relations adviser, said: “The strain put on the health service in recent years, with jobs having been cut and resources frozen as patient numbers have gone up, has made the situation even worse.”

NHS Employers, which represents hospitals and other providers of care, estimates that over 30% of all NHS sick leave is caused by stress, which is believed to cost the service up to £400m a year in lost productivity and the cost of replacing stressed staff. The situation led the organisation to publish new guidance for NHS managers in November about how to reduce stress and help staff.

In 2012 and 2013 combined, 38% of NHS personnel told a staff survey that they had suffered stress in the previous year. An NHS Employers spokesman said there had been “a lot of work and progress in this area”. The Department of Health maintained that stress levels were stable across England. “We know nurses are working extremely hard, but these figures create a misleading picture,” a spokesman said. “The NHS staff survey shows job satisfaction has risen in the past three years while work-related stress has remained stable.” There were 6,000 more nurses in the NHS in England than when the coalition took power in 2010, he added.