Maternity imageMaternity units are having to shut temporarily or turn away expectant mothers because NHS childbirth services have too few midwives and are struggling to cope with the ongoing baby boom, according to a hard-hitting official report.

Parts of the maternity service in England are becoming overstretched at times because it is short of 2,300 midwives and births are at their highest level for 40 years, according to the National Audit Office, the government's spending watchdog.

Between April and September 2012 a total of 28% of maternity units closed for 12 hours or more, including eight that shut for a total of at least two weeks, either because they lacked physical capacity or midwives.

"Where the demand for maternity services might outstrip capacity, some trusts are restricting access through pre-emptive caps on numbers or reactive short-term closures in order to safeguard the quality of care," the report says.

The watchdog found that although most of the almost 700,000 women a year who give birth in England were happy with the service they received, services in many places still needed to instigate major improvements.

Mistakes in maternity care account for a third of the £1bn a year the NHS has to spend settling medical negligence claims. The number of lawsuits involving alleged failings in maternity care shot up by 80% in the five years to 2012/13, obliging the NHS to set aside £482m to cover the costs of those claims. The NAO says nearly one-fifth of all spending on maternity services is for clinical negligence cover.

"It is absolutely scandalous that one-fifth of all funding for maternity services, equivalent to around £700 per birth, is spent on clinical negligence cover," said the Labour MP Margaret Hodge, who chairs the Commons public accounts committee.

She said that figure underlined that the system was not working as it should and ministers needed to ensure improvements were made. Settlements in cases involving a baby who suffered brain damage at birth because of blunders by maternity staff can cost as much as £9m each in damages.

The NAO's assessment of maternity care since 2007 highlighted improvements such as the increased number of women who now have either an obstetric unit or one of the growing number of midwife-led units within half an hour's drive.

But it also found that in England in 2011 one in every 133 babies was stillborn or died within a week of birth – a higher rate than in Scotland, Wales or Northern Ireland. Hodge said she was "shocked" by that finding.

The NAO found that while 73% of obstetric units in hospitals had a consultant on duty for at least 60 hours a week, up from 8% in March 2007, more than half (53%) – including all the largest ones – did not provide as much consultant cover as recommended by the Royal College of Obstetricians and Gynaecologists.

Cathy Warwick, chief executive of the Royal College of Midwives, said the report vindicated the union's persistent warnings about under-staffing and said it should act as a wake-up call to ministers and bosses of maternity units to invest more money to help ensure services were safe.

Labour accused David Cameron of breaking his high-profile pledge to increase midwife numbers by 5,000.

The health minister Dr Dan Poulter, who is also an obstetrician, said the NHS had 1,300 more midwives than at the 2010 election. He also pointed to the fact that 5,000 more had been in training since then – though the NAO said a high dropout rate and impending retirements could mean that shortages continue for some time to come.

Poulter said the number of midwives working in the NHS was increasing twice as quickly as the birthrate, and the presence of consultant doctors on maternity wards had increased significantly. "The work we are doing is making a big difference to the experience that mums and families have of NHS maternity services, with more choice and a better environment where women can give birth," he said.

Belinda Phipps, chief executive of the NCT, the childbirth and parenting charity, said: "Midwives are being overworked, maternity units are understaffed and as a result parents are suffering."

An NCT report said 88% of women had not met the midwife who cared for them during labour and birth before they had gone into labour, and 60% of women failed to receive enough postnatal support.