Midwives are delivering almost 25% more babies than experts believe is appropriate, figures released by the Conservative Party suggest.
The birth rate in England has risen sharply
The Tories say the government's failure to anticipate a big rise in the birth rate in England has left midwives under intense pressure.
They argue this could derail ministers' commitment to offering all women a choice of where to give birth by 2009.
But ministers dismissed the claim as "scaremongering".
Since 2001, the number of live births in England has increased by almost 71,000 (12.5%), and is now at a 26-year high. In 2006, 635,679 live births were recorded.
However, the increase in the number of midwives has not kept pace, rising by 4.5% over the same period.
The Royal College of Midwives recommends that each midwife should deliver 27.5 babies a year.
However, the surge in live births meant that on average each midwife in England delivered 33.7 babies last year.
The Conservatives said government experts had under-estimated the actual number of live births last year by nearly 40,000.
They said the intense pressure being placed on midwives explained why the rate of home births - which are time consuming and labour intensive - is relatively low.
Units under threat
Shadow health secretary Andrew Lansley also argued a shortage of midwives was jeopardising the future of some maternity-led units as midwives were being deployed to larger consultant-led units to ensure safe staffing levels.
He said: "Labour's plans for maternity services are in chaos. Their flagship policy to offer choices to expectant mothers over where and how they give birth is underpinned by assumptions about the number of births which are wide of the mark.
"Labour are trying to pursue a policy for which they do not have the staff. We are 3,600 midwives short and yet the number of midwives is going down and newly trained midwives can't find jobs."
Dame Karlene Davis, general secretary of the Royal College of Midwives, said a survey of heads of midwifery carried out by the college last month showed three-quarters had seen an increase in birth rate on their units, and over 80% did not have staffing levels they needed to cope.
She said the problem would be compounded by the fact almost 45% of midwives were set to retire over the next decade.
She said: "We have seen too many service cuts, too many midwives lost, and too many mothers and babies getting a service that should shame the fourth richest country in the world.
"Action is needed, and needed now, to put the resources and effort needed into maternity services."
A Department of Health spokesperson accused the Conservatives of "irresponsible scaremongering".
"There are now 2,423 more midwives than there were 10 years ago.
"The number of students entering the profession has risen by 44%. And 80% of women are happy with the maternity services they receive. This is hardly a picture of 'chaos'. "