Charing cross hospital has become 'incapable of providing major acute patient care', according to council leader Stephen Greenhalgh.
He launched his stinging attack after the hospital, run by Imperial College NHS Trust, axed its vascular surgery ward and sent it to St Mary's Hospital, Paddington and then threatened to remove all its orthopaedic surgeons.
Mr Greenhalgh, whose father, Professor Roger Greenhalgh, is head of the Imperial College Vascular Surgery Research Group, fears St Mary's doesn't have the necessary beds capacity to cope with the change.
"I am very concerned," he said. "We have been provided with no evidence that this move will lead to improved patient care or that St Mary's will be able to cope with the demand for beds."
The plans to move orthopaedic surgeons to St Mary's, possibly in the summer, has also come under attack from Mr Greenhalgh.
He said: "Charing Cross is being downgraded to a local hospital incapable of providing major acute patient care as it has done for nearly 40 years."
The vascular unit was shut after the outbreak of a bacterial infection in December, and was never reopened.
Imperial College NHS Trust says the merger will benefit patients from all over north-west London and insists some vascular treatment will remain at Charing Cross.
A spokeswoman said: "The trust will continue to provide routine vascular services at Charing Cross, including varicose vein treatment, outpatient consultations and diagnostic tests."
She added moving the surgeons would mean more surgical proceedures which would 'result in better outcomes for patients', but Mr Greenhalgh branded that claim 'nonsense'.
"The idea that these moves will improve patient care is nonsense and the evidence that clinical outcomes will improve is also lacking," he added.
Claire Perry, managing director of the trust, said the changes would not 'materially affect the care offered at Charing Cross', adding: "The proposed changes are designed to improve our patients' experience and their clinical outcomes."