Maternity units are being forced to close their doors more often as the NHS comes under increasing pressure, the Royal College of Midwives (RCM) has warned.
A shortage of midwives is having a major impact and mistakes "will almost certainly be made", with patients put at risk.
Budget cuts are squeezing services as midwives also struggle to cope with a rising birth rate and increasingly complex births, the union said.
A poll of 83 heads of midwifery at NHS trusts found a rise in the number of units that had closed their doors to mothers in need.
Some 42% of units closed in the last year because they could not cope, up on the 33% the year before.
Units closed their doors on 6.6 separate occasions on average in 2014 and 4.8 separate occasions in 2015.
The most times a single unit closed was 23 times in 2015.
More than 90% of the senior midwives said their unit was dealing with more complex cases than the previous year, while 30% said they did not have enough midwives.
One in 10 (11%) had to reduce services in the last year, including offering fewer parent classes as well as less support for bereaved parents and help with breastfeeding.
Services such as home births and postnatal care are also suffering as staff working in the community are called in to cover gaps in hospitals, the RCM said.
Midwives also have to leave postnatal wards - where aftercare to spot serious infections and breastfeeding support are offered - to help in labour and delivery suites.
Three-quarters of senior midwives said they had to redeploy staff to cover essential services either very or fairly often.
Many said they were unable to do their job to a standard they were happy with, while one in five had reduced staff training owing to other demands.
Cathy Warwick, chief executive of the RCM, said: " All of this shows a system that is creaking at the seams and only able to deliver high quality care through the efforts and dedication of its staff.
"When services are operating at or beyond their capacity, safety is compromised and mistakes can, and almost certainly will be made, through no fault of the dedicated staff delivering the service.
"The Government is responsible for this and it is they who are letting down women, babies and their families, as well as the staff they purport to value. This is simply not acceptable."
One head of midwifery who responded to the survey said: "All staff including non clinical based roles are under extreme pressure with many of us working 60 hours per week on 37.5 hours contracts for no additional pay and still not keeping up.
"When struggling, the response from the top floor is to delegate. To whom I ask? Everyone is too busy."
Another said the pressure staff were under was "palpable", adding: "Staff who do not have time to train, develop professionally and do not feel valued will struggle to provide safe, high quality compassionate care."
According to the RCM, there is still a shortage of 2,600 full-time midwives.
Dr Clare McKenzie, vice-president for education at the Royal College of Obstetricians and Gynaecologists (RCOG), said: "The results of this survey are extremely worrying. Stretched and understaffed maternity services affect the quality and safety of care provided to mothers and babies, and restricts the choices available to women.
"On the whole the UK is a safe place for women to give birth but pressure on maternity services is growing, placing stress on doctors, midwives, managers and patients.
"Pregnancies are increasingly complicated due to the rising levels of obesity among the population, along with increasing numbers of first-time older mothers and multiple pregnancies."
A Department of Health spokeswoman said: "We're determined to make sure every mother and baby gets the highest quality care no matter where they live.
"We've invested in 1,900 more midwives and 3,600 more health visitors since 2010 and NHS England has commissioned a major independent review of maternity services for women and babies across the country."