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10 June 2015 Last updated at 09:08 BST
Dr Gideon Lauffer is alleged to have misdiagnosed a man with acute kidney failure at the West Middlesex Hospital in Isleworth after allegedly misreading blood tests.
The patient, Nick Johnson, was sent home and told he was suffering from constipation, but he was later re-admitted to hospital and treated for blood cancer.
Dr Lauffer had been a consultant surgeon and was found guilty in 2010 of serious professional misconduct and dishonesty. He then retrained as a non-surgical hospital doctor.
Dr Lauffer said: "Following an extensive investigation, the General Medical Council decided to close the case with no action on my registration.
"I owe obligations of confidentiality to patients and it would be inappropriate for me to comment further."
The General Medical Council said Mr Johnson's complaint could not be taken forward due to a lack of evidence.
But Mr Johnson, who is still recovering from cancer treatment and is awaiting the removal of his gallbladder, said he is considering how he can take the case further.
BBC London's Ayshea Buksh reports.
A doctor who was suspended as a surgeon for carrying out operations "beyond his competence" five years ago is at the centre of new concerns about his treatment of patients.