Health visitors are struggling to care for families properly because they have “dangerously high” workloads in which some are looking after as many as 829 children, a study shows.

Falling numbers of health visitors mean that in most parts of England they are now looking after more – often many more – under-fives than the recommended maximum 250.

The findings have prompted warnings that health visitors are so busy that they may fail to spot child abuse, domestic violence and postnatal depression and have too little time to help mothers bond with their babies.

Health visitors are nurses or midwives who have switched roles to focus on supporting families with young children. Ministers have praised their work as “absolutely crucial” in ensuring babies and infants get a healthy start in life and they undertake a series of important checks before and after childbirth.

However, research by Dave Munday, a health official with the union Unite, has found that health visitors in the London borough of Hounslow have average caseloads of 829 children under five – the highest in England. Similarly, those in Luton and Peterborough are each looking after 756 children of that age, while in Croydon, south London, it is 591.

Those are far above the 250 children per health visitor limit that the Community Practitioners and Health Visitors Association, a part of Unite, and the Institute of Health Visiting both say is the maximum caseload if care is to be safe and of a high quality.

“These findings are very worrying”, said Munday, who obtained the data under freedom of information laws. Since 2015, local councils have commissioned health visiting services, but most pay NHS trusts to carry out the work. It should be a “significant concern” that only 18 of the 132 providers that supplied figures had average caseloads under 250, while in 18 others caseloads involved 500 or more children, Munday added.

Last month the Care Quality Commission, which regulates the NHS, warned Birmingham Community Healthcare NHS trust that it was concerned about aspects of its performance, including that its health visitors were dealing with, on average, 500 families each.

Richard Kirby, the trust’s chief executive, told its 5,000 staff that the CQC had voiced alarm about “the pressures facing community children’s services and especially health visiting and the risks that these posed to our ability to provide safe care for children and families”. The trust has begun “an extensive programme of improvement” and case loads now average 440, a spokesman said.

Sarah Carpenter, Unite’s head of health, said: “With high case loads there are increased risks of baby deaths from poor health or abuse, of maternal mental health issues that unspoken about could lead to attempts at suicide, and undiscovered domestic violence with trauma for all involved. The inability of those staff left in the demoralised health visiting service to deliver the service that they want and need to deliver leads to horrific longer-term problems for families, and a longer-term cost for the NHS as it struggles to repair the damage caused by lack of early intervention.”

Health visitors in Brighton and Hove have the smallest case loads in England, with each one responsible for just 73 under-fives. Blackpool has the second smallest, with 87, followed by Wakefield with 101.

The number of health visitors rose from 7,849 in September 2010 to 10,236 in September 2015 because the coalition government made recruitment a priority. But that fell to 8,497 in September last year and again to 8,016 in May this year, as councils cut spending on public health after their grants for that purpose from central government were reduced.

A Department of Health and Social Care spokesman said: “Health visitors play a vital role in supporting parents and babies across the country. That’s why NHS England, NHS Improvement and Health Education England are working with trusts on a range of recruitment, retention and return-to-practice programmes to ensure that the required workforce are in place to deliver safe and effective services.”