Maternity services are close to breaking point and care for mothers is worsening, the UK's leading midwife warns in a dramatic plea over the declining state of childbirth on the NHS.

Labour wards are struggling to give women the proper quality of care under the "relentless" pressure of a record birth rate, staff shortages and increasingly complex births, says Cathy Warwick, general secretary of the Royal College of Midwives.

Writing in the Observer, Warwick warns that the situation in maternity units is so grave that the safety of women giving birth is under threat. Midwives, she says, "are deeply anxious about the care being delivered. They believe that the service they are giving to women and babies is deteriorating and that safety is too often being compromised. The service is teetering on the brink; the cracks are beginning to appear."

Midwives "are on the edge of our capacity to cope", with extremely busy days, once the exception, now the norm, she adds: "Midwives cannot carry on working like this day in and day out, often without breaks, and continue to practise safely." There are too few midwives to ensure that every delivery is as safe as it should be, she warns.

Warwick's remarks have prompted fresh concern about whether ongoing shortages of midwives in many units is affecting the experience of the 98% of women who have their children in hospital. They come amid rising concern about maternity safety. Yesterday the president of the Royal College of Obstetricians and Gynaecologists, Dr Tony Falconer, claimed that the NHS offered an inferior service to women who give birth at night to those who deliver during the day, when staff on duty make better decisions because they are more experienced.

And last month the Care Quality Commission, the NHS watchdog in England, discovered that 22% of mothers were left alone during labour and found that frightening, a situation experts say is due to midwives having to look after several women simultaneously and so not providing what should be one-on-one care.

Staff shortages are so acute that midwives who should be assisting home births and working in the community are instead being forced to help out in hard-pressed hospital units, and post-natal care is also being hit, says Warwick. "Maternity services are not coping and are under assault," she adds.

She accuses David Cameron of backtracking on a high-profile pledge made before last year's election to hire 3,000 extra midwives to help relieve staff: "Despite repeated and persistent requests from us for his government to honour this pledge, they will not." The failure of midwife numbers to keep pace with the spiralling birth rate of recent years means the NHS now needs another 3,500 midwives to guarantee safe and proper care, in Warwick's view.

Belinda Phipps, chief executive of the National Childbirth Trust (NCT), said Warwick had rightly drawn attention to the link between midwife levels and patient safety. "Cathy Warwick is not crying wolf; she is not exaggerating the situation," she said. "All good practice says: one midwife to one woman in labour. But we too often have one midwife running between three women in labour. If you are sharing one midwife between three women, she may not spot a problem arising as soon as it should be spotted. Many of the women who come to our NCT reunions after giving birth tell frightening stories. Some are very traumatised, others have been badly treated and some end up with postnatal depression."

Jane Sandall, professor of midwifery and women's health at King's College London, said: "Midwives are saying that they are coping with a workload that a few years ago would have been intolerable. When you don't have enough staff it has implications for safety. Problems affecting the mother or the baby can get missed, which could make it more difficult to arrange a fast response – for example, to a baby's heart rate dropping."

John Healey, the shadow health secretary, said that Warwick had given "a very serious warning about the safety and standards of NHS maternity services, which help nearly 800,000 babies born each year.

"[Her] comments should ring alarm bells for the government because they show Britain's midwives believe David Cameron is breaking another NHS promise to improve maternity care, and frontline NHS staff are losing confidence in [health secretary] Andrew Lansley," he added. Improvements to maternity care under Labour were now at risk of going backwards, Healey said, adding: "2011 could be the Year of Tory cuts to the NHS, as people start to see pressures in the NHS hitting services to would-be mothers and other patients."

The Department of Health stressed that mothers' and babies' welfare was vital. "Across the NHS there must be a culture of patient safety above all else. All mothers should expect consistently excellent maternity services," a spokeswoman said.

Ministers plan to extend choice of place of birth for mothers between a home birth, midwife-led care and a hospital obstetric unit, and a record 2,493 would-be midwives are in training, which will lead to "a sustained increase in the number of new midwives available to the service over the next few years".

Professor Steve Field, former chairman of the Royal College of General Practitioners, said the new maternity networks which will be set up in England as part of the government's contentious NHS reforms should improve the quality of care. "Maternity care is not as good as it should be," he said. "But GPs, working with obstetricians and patients themselves, should be able to design better patient-centred services."