More than a third of maternity units have been forced to shut their doors to labouring women because they could not cope with demand, leading midwives have warned.
New figures from the Royal College of Midwives (RCM) show that 38.6% of maternity units had to temporarily shut during the last year.
A new poll conducted among senior midwives revealed that units closed their doors on 281 separate occasions.
The RCM said the average unit temporarily closed eight times, but one unit shut its doors 50 times.
Eight units had to close their doors on 10 or more occasions.
The College, which is holding its annual conference in Harrogate, said the number of closures was a reflection of the rising demands on services as well as increasingly complex births and issues with staffing levels.
The poll, which was completed by 53% of the heads of midwifery from around the UK, found that nine in 10 believed their unit was dealing with more complex cases than last year.
Meanwhile, almost two in five said that they did not have enough midwives to cope with the demands on the service.
And 19% of the 85 senior midwives who responded to the poll said that their budget had decreased in the last year.
One in 10 said that they had to reduce services in the last year, including reductions in parent classes and breast feeding and bereavement support.
Four in five also reported that they had to redeploy staff to cover essential services - meaning staff who were supposed to be delivering antenatal care and community care were redeployed to cover labour delivery suites.
"Yet again we are seeing senior midwives describing services that are being battered by increasing demands, inadequate resources and staffing shortages," said Cathy Warwick, RCM chief executive.
"It is very often only through the hard work, goodwill and sacrifice of maternity staff that services are able to deliver the safe and high quality care women need.
"These results show that midwives have never been so challenged in their ability to continue to provide high quality care for women and their families."
She added: "This situation isn't sustainable and the Government must start to invest in NHS staff because we all know that an investment in staff is an investment in high quality, safe care."
Meanwhile a separate poll from the RCM, released on Wednesday, revealed that inadequate staffing levels were driving midwives to leave the NHS.
The study of more than 2,700 midwives found some were working in "dangerous" conditions, having 12-hour shifts with no break and worried about making "tragic" mistakes, with some looking after as many as 15 mothers and babies at a time.
Ms Warwick criticised the Government's "disastrous policy of pay restraint in the NHS ", adding: "Midwives and maternity support workers are working harder than ever at the same time as they have had six years of real-terms cuts to their pay.
"We want to see a cost of living increase for midwives, maternity support workers and other NHS staff to show them they are valued and to reward their hard work.
"There is a growing shortage of midwives and midwives tell us because of the demands they face they are intending to leave midwifery, making the shortage worse."
Labour's shadow health minister, Justin Madders, said: "This shocking poll is yet more evidence of the crisis engulfing our NHS as a result of Tory underfunding.
"Six years ago the Tories promised 3,000 more midwives, but they failed to deliver them. This has left maternity units across England operating without enough staff and unable to cope with rising demand."
A Department of Health spokesman said: "We want midwives to feel that they have enough time and support to give good care.
"That's why there are over 1,800 more midwives on our maternity units since 2010, as well as 6,400 currently in training, with our changes to student funding creating thousands more training places by the end of this parliament.
"The NHS is also launching a £5 million campaign to help improve the health and well-being of staff, and as the health secretary has repeatedly made clear, overseas workers form a crucial part of our NHS and we value their contribution immensely."