The Tories say maternity units in England closed 533 times in 2008
More hospitals in England had to turn away women in labour last year because they were full, figures suggest.
Nearly half of 104 trusts responding to a survey by the Tories said they shut their maternity units at least once and diverted women to other hospitals.
The total number of closures for 2008 was 553, up from a total of 402 in 2007 when a similar survey was carried out.
The Department of Health said a record amount of money was being invested in maternity care.
Many of the trusts, while admitting shutting their doors at times, have said that the women were accommodated in other units nearby.
Andrew Lansley, shadow health secretary, said: "These figures are a telling reminder of Labour's terrible record on maternity.
"Every one of these figures tells an awful story of mothers being turned away from hospital at a hugely emotional time.
"Labour seems to be deliberately running down maternity services in some hospitals as a precursor to shutting down maternity units altogether.
"This is completely against the best interests of mothers and their new babies, and denies them the choice they deserve."
Jacque Gerrard, director for England for the Royal College of Midwives, said: "Capacity within maternity units is being stretched to the limit and beyond, resulting in closures.
"This level of reported closures is deeply worrying, leaving exhausted midwives having to turn women away from the maternity service, and women being let down.
"The Royal College of Midwives is urging people who run health services locally to be more proactive and use money earmarked for maternity services actually for maternity services, so that women are not being left worried and deeply disappointed."
The Tories said of the 104 out of the 148 NHS Trusts in England that responded to the survey, 14 were forced to close their maternity unit more than 10 times.
And they said some of the highest number of closures were at hospitals which are closing maternity units because of reorganisations of their services.
They said Barnet and Chase Farm Hospitals NHS Trust is consulting on changes to its midwifery service and had 91 closures last year.
But a spokesman for the Trust said: "We manage our maternity services as one unit and all women are advised at their initial booking that they may be asked to attend the other site if their preferred one is working at full capacity.
"Between 1 January and 31 December 2008, there were 63 occasions on which mothers were diverted from Barnet Hospital to Chase Farm hospital and 28 when mothers were diverted from Chase Farm to Barnet.
"There has been no occasion when the entire service has been closed to admissions."
And a spokesman for Maidstone and Tunbridge Wells which had 97 closures in 2008 said: "We have two nationally acclaimed maternity units that work in unison - if one is full women are diverted safely and routinely to the other.
"Very rarely are our services ever closed.
"We have had only one complaint from the small number of women who have been diverted from one unit to the other in the past year.
"Pregnancy is not entirely predictable, but if lots of babies do come at once we have the facilities to cope."
East Sussex Hospitals NHS Trust who had 75 closures admitted that there had been occasions when their maternity units had had to divert women in labour: "Although this is disruptive to the individuals affected, we make no apology for putting mothers and babies safety first.
"This is a national problem and it needs to be recognised that neighbouring NHS Trusts also find it necessary to divert patients from their maternity units from time to time."
Belinda Phipps, chief executive of the National Childbirth Trust, said: "It is terrifying to go into hospital in labour and to find the doors are closed. Women rely on maternity services being open for them twenty four hours a day, seven days a week.
"To prevent further closures, we want more funding for maternity from government, particularly funding for more midwives, and to see better planning locally.
"As birth rates have risen it is obvious their services are going to feel the pressure. The number of pregnant women is predictable and must be anticipated and responded to in advance."
A spokesman for the Department of Health said: "Sometimes units do have to temporarily shut their doors, usually for very short periods of time.
"We appreciate that it is distressing to be told that your care is going to be provided elsewhere but this is always undertaken in the interests of safety for the mother and baby.
"We are also investing more money in maternity care than ever before.
"The NHS is well on course to recruit an additional 1,000 midwives by September 2009, and we expect to see up to 4,000 by 2012."