Paramedics are taking tens of thousands of days a year off sick with stress, as growing numbers of 999 calls add to the pressure on NHS ambulance services.

The number of days being lost to paramedics having time off work because they are struggling with stress, anxiety or other mental health conditions is rising, official figures show.

Statistics have revealed that paramedics working for seven of England’s 10 NHS regional services have been signed off sick with such ailments for 183,962 days in the last four years.

In all, 35,872 days were lost for that reason in 2013-14; that went up to 41,412 in 2015-16. Figures for the first nine months of 2016-17 suggest last year’s total will be even higher.

Paramedics and health unions claim staff shortages, pressure to meet 999 response targets, routinely long shifts lasting up to 15 hours and the emotional toll of dealing with sick patients and their families are behind the trend.

There is concern that stress leave is exacerbating the existing pressures on ambulance services’ ability to respond fast enough to the increasing number of emergency calls at a time when vacancies and early retirement are widespread.

“Paramedics provide life-saving care, often in stressful circumstances, but the blue lights flashing right now are for the ambulance service. It is unacceptable that such high levels of stress are now seen as part of normal life for ambulance staff,” said Tim Farron, the leader of the Liberal Democrats, who obtained the figures under freedom of information laws.

“The things paramedics see and have to deal with on a daily basis must keep people up at night. We need to do more to care for those who care for us”, he added.

The London ambulance service has lost the most paramedic days through stress. It lost 11,911 days to it in 2013-14 but that rose to 12,215 in 2015-16 and then again to 14,447 in the first nine months of 2016-17 alone.

The South-East Coast ambulance service’s figures rose from 5,659 to 6,366 figures, and the South West’s from 4,162 to 5,228, over the same period. Only the West Midlands service saw a significant fall in its numbers in that time.

“Ambulance services are haemorrhaging staff, and struggling to hire new recruits. That puts extra pressure on those left behind with crews having to work very long shifts,” said Alan Lofthouse, Unison’s national ambulance officer, who is a former paramedic.

“Rising demand on 999 services means patients wanting not just emergency treatment but also medical care as they can’t see their GPs. Overstretched A&E departments with long waits to hand over patients, abuse and threats of violence from motorists, relatives or under the influence casualties. It’s no wonder ambulance staff are having to take time off for stress.”

The true figures for England as a whole will be a lot higher as the North West, Yorkshire and South Central ambulance services did not provide any figures.

A survey of ambulance staff undertaken by the union Unite last year found that 89% of ambulance staff said that morale and motivation in their workplace was falling and 88% identified stress as the main reason for that. In the same poll 91% of the 362 ambulance crews questioned said their workloads were growing and 85% said they worked beyond their contracted hours.

Meanwhile, Labour claims NHS England’s plan to significantly relax the requirement on hospitals to treat 92% of patients waiting for an operation within 18 weeks may be illegal.

Labour has challenged the health secretary, Jeremy Hunt, to set out the legal basis for dropping a commitment on waiting times that is enshrined in the NHS constitution.

The move, announced on Friday, prompted widespread criticism from medical groups.