A banker, who fantasised about becoming a surgeon, is said to have worked 14 months in a major hospital, and taken part in 190 operations before he was discovered.
Christian Eberhard, 30, forged medical papers and wrote out his own degree with a fountain pen, claiming to have qualified at Oxford University.
He even wrote himself glowing references from doctors who existed only in his imagination.
Medical authorities at the University Clinic of Erlangen in Germany never questioned his "training" in Britain, even though he spelt doctor with a K on his bogus qualifications and misspelled medicine.
A German court heard Eberhard became "obsessed" with the idea of becoming a doctor after he worked in a hospital for 10 months in lieu of compulsory national service.
When his parents refused to support him financially for the eight years he would need to study to qualify to practise medicine, he became a successful securities expert, but ditched his banking career to indulge in his medical fantasies.
He downloaded forms from the Internet and told medical authorities that he had qualified at Oxford.
Eberhard then spent two years training in surgery techniques at the Erlangen hospital before he was then promoted to an assistant surgical doctor.
He became involved in highly complex procedures, including spinal, liver and lung operations.
Last November he was jailed for three years but the prosecution appealed and he has been given an extra six months in prison.
At his trial, Werner Hohenberger, director of the hospital, said Eberhard was "diseased" and "a high-grade pathological liar with enormous criminal energies".
"Hundreds of people have done this in the past and probably will continue to do so," the Daily Express quoted Hohenberger as saying.
Acknowledging patients' lives had been at risk, he said none had died or suffered as a more senior doctor always supervised Eberhard.
"I am proud of what I accomplished nonetheless. Less legs were amputated when I was there because I knew what I was doing," Eberhard said at his latest court hearing.