Thousands of patients from Sutton are expected to be treated at Epsom Hospital's accident and emergency (A&E) department if St Helier Hospital's A&E closes.
NHS SW London's Better Services, Better Value Review (BSBV) is proposing St Helier Hospital's A&E, maternity, and children's wards close by 2016-17, so other accident and emergency departments across SW London can be improved.
But figures released by BSBV this week show 23 per cent of patients - or 8,889 patients- who would have used St Helier's A&E are predicted to go to the Surrey hospital instead if the planned closures go ahead, meaning those patients would not benefit from the A&E improvements being used by BSBV to justify the St Helier closure.
Epsom's A&E would have to expand to cope with demand.
The hospital estimates its A&E could currently deal with another 6,700 patients a year.
The news has sparked further debate over how far people would travel for their A&E and maternity services, and whether predictions being modelled by NHS SW London to justify their proposals are reliable, and would make their plan financially viable.
Councillor Mary Burstow questioned whether BSBV could sell its proposals to Sutton residents that they will recieve improved care in SW London, when it was predicted thousands would not benefit from those improvements.
She said: "It is clear many people in Sutton, Worcester Park, Cheam and other areas will want to use the hospital that is closest to them, and that will be Epsom. That may be even more than NHS SW London are predicting."
MP Paul Burstow questioned the reliability of the predictions about patient movements being made by BSBV, and how people using different hospitals than those they were expected to could affect the financial viability of service models and financial predictions being in SW London.
He said: "People may vote on the proposals with their feet."
He said: "It is also clear money for treating patients will then be moving out of NHS SW London into Surrey."
A BSBV spokesman said NHS Surrey were represented on the BSBV programme board and fully understood its predicted increase in patients.
He said: "The money follows the patient, so if more patients are treated at Epsom, then the hospital will receive more funding.
He added:"What is clear is that local doctors and nurses believe their proposed model of emergency care, with three improved emergency departments in south west London, would save more lives and improve services for patients.
"Travel times matter, but what is most important is seeing the right clinical team with the right medical equipment.
"Obviously, in a medical emergency a blue-light ambulance would take patients to the A&E that could help them the most."