Concerns have been raised over St George's Hospital's ability to cope if proposed frontline closures at St Helier go ahead, after it emerged the Tooting hospital is missing waiting list targets.
The letter leaked to the Sutton Guardian reveals that in some cases at the Tooting based hospital, patients have breached the national 18 week referral to treatment waiting time target.
It asks SW London GPs not to refer patients to St Georges specialities including upper GI surgery, bariatric surgery, spinal surgery, Ear Nose and Throat and gynaecology.
Gynaecology is one of the departments likely to be lost from St Helier if its maternity department is closed.
As part of a healthcare review, Better Services Better Value, it was recommended that St Helier Hospital should be the hospital in SW London to lose its accident and emergency and maternity departments. This would mean St Helier's A&E and maternity patients would be sent to St George's.
The letter from the medical directors of NHS SW London has raised concerns surrounding the hospital's ability to cope with the future changes.
It states that the problems at St George's date back to the introduction of a new patient reporting system at the trust and difficulties are compounded because the hospital is busy.
The letter advises all GPs in SW London to refer their patients in five different specialities to alternative providers including St Helier until January 2013.
Tom Brake, the MP for Carshalton and Wallington, said: "How can we afford to lose the services at St Helier and for it to become a centre for planned surgery, sending extra patients to St George's, when St Helier is being used to carry George's case load?"
A spokesperson for BSBV said: "Under the BSBV proposals, St George’s Hospital would be expanded - it would have larger A&E and maternity services and would host a new children’s hospital for south west London. It would be more than able to cope."
A spokesman for St George's Hospital said: "A plan has been developed with our clinical commissioners to help us address the short term capacity issues the trust currently faces with a few very specialist treatments which are very popular with patients and GPs from across south west London and Surrey. This includes working with other NHS providers and asking GPs to consider referring their patients to these other local hospitals that have spare capacity for these specialities over the next few months.