A hospital maternity ward that would take on hundreds more births a year if proposed closures at St Helier Hospital go ahead was forced to shut for 30 hours because it was too busy.
Concerns have been raised over St George's Hospital's ability to cope with increased demand after it emerged the Tooting hospital closed its maternity department from 1pm on Friday, August 25 to 7pm the next day.
As part of a healthcare review, Better Services Better Value, has recommended St Helier Hospital should lose its maternity department.
This would mean more than 3,000 St Helier expectant mothers would instead be sent to the other hospitals in the region, including St George's.
Paul Burstow the MP for Sutton and Cheam, said: "The fact that the maternity unit at St George's closed its doors this weekend is yet another reason why those driving plans to close St Helier's maternity unit should think again. The birth rate is rising, and as every parent knows, babies don't always arrive when we expect.
"St Helier, where my children were born, offers a fantastic service and it makes no sense to cut capacity, putting even more pressure on already overstretched maternity units."
Tom Brake, the MP for Carshalton and Wallington, said: "The temporary closure of St George’s maternity underlines the fact that today, the NHS could simply not cope with closing the maternity at St Helier Hospital.
"The NHS is under pressure from all sides, with more people going to A&E and more mums giving birth.
"Why would you shut a highly performing A&E and maternity unit when you know they are in great demand?"
St George’s maternity unit delivers over 5,000 babies a year and is a specialist service for the most complicated pregnancies in the region.
A Better Services Better Value spokesperson said: "Our proposals for maternity services are based on expanding the maternity units at St George’s, Croydon and Kingston Hospitals, so they would be larger units.
"Our projections show they would be comfortably able to cope with the projected numbers of births in years to come.
"Our proposals would ensure that all births were overseen by senior doctors and that patients got one to one midwife care, meaning the services would be safer and meet national best practice guidance.
"These changes would not happen overnight, but would be implemented over several years."
A spokesperson for St George’s Healthcare Trust said: "We saw a particularly high number of these complex patients over the weekend, all of whom required extra specialist support.
"To ensure we could maintain a high quality and safe service, we closed the unit to additional patients for a short period of time."