The Government must address the "serious shortage" of midwives, the Royal College of Midwives said, as an independent report found that numbers of staff are one of the main challenges facing maternity services.
Long-term vacancy rates for midwives have steadily increased in recent years, the Care Quality Commission (CQC) report found.
Although births in England increased by over 21% between 2001 and 2010, the number of midwives only increased by around 15%, from 18,048 to 20,790.
Louise Silverton, deputy general secretary of the Royal College of Midwives, said: "This supports and highlights all we have been saying for many years about the shortage of midwives and the need for serious investment in maternity services.
"A failure to have adequate numbers of midwives leads to mistakes and lower quality care.
"We recognise investment in midwifery training but this will be wasted if the newly qualified midwives cannot find jobs. Once qualified, they need support from experienced midwives whist they consolidate the skills learned during their training. Many of these midwives are those whose jobs are currently under threat.
"This Government must take this issue much more seriously and give it more attention than they are doing. If they do not mothers, babies and their families are the ones who will suffer the consequences of this Government's failure to ensure that maternity services have the resources to meet the demands facing them."
The union has launched an online petition calling for 5,000 more midwives in the NHS in England.
A Department of Health spokeswoman said: "Local healthcare organisations, with their knowledge of the patients they serve, are best placed to decide how many doctors, nurses and other health professionals they need to deliver high quality maternity services.
"The ratio between midwives and the birth rate now is similar to the rate 10 years ago, both have increased by around 17%. The birth-to-midwives ratio does not indicate the safety or quality of service provided."
The CQC report, which is based on findings from unannounced inspections of 14,000 health and social care providers in England, found that across the board one in 10 institutions were failing to meet staffing standards.
"The non-availability of temporary staff and organisations leaving vacancies open for a number of months - particularly for qualified staff - can lead to compromises in the quality of care given to people, and staff training and supervision," the CQC said.