LONDON—A British surgeon has admitted to assaulting two patients by burning his initials into their livers during transplant operations.

Simon Bramhall pleaded guilty Wednesday to two counts of assault, in a case a prosecutor called “without legal precedent in criminal law.”

Prosecutor Tony Badenoch said the branding was “an intentional application of unlawful force to a patient whilst anaesthetized,” and was an abuse of Bramhall’s position.

Bramhall used an argon beam coagulator, which seals bleeding blood vessels with an electric beam, to mark his initials.

Bramhall resigned from the Queen Elizabeth Hospital in Birmingham in 2014 after facing a disciplinary hearing when another doctor discovered the branding.

Bramhall, who is 53, was released on bail. He is due to be sentenced Jan. 12 at Birmingham Crown Court in central England.