(NewsNation) — A British doctor awaits sentencing for attempted murder after he arranged a fake home visit for his mother’s partner, whom he injected with poison while wearing an elaborate disguise.

Fifty-three-year-old Thomas Kwan pleaded guilty Monday to attempted murder. He previously denied the allegations, saying he intended to cause “no more than mild pain and discomfort,” the BBC reported. He’s scheduled for sentencing on Oct. 17 and could face “substantial” time in prison, the judge said.

Kwan — a general practitioner from North Yorkshire — was eyeing his estranged mother’s inheritance when he put the plan into action, local police said. The two had a falling out when Kwan learned his mother planned to leave her Newcastle home to her partner of more than 20 years, the BBC reported.

Kwan sent several fake medical letters to the man claiming he was due for a home medical check-up and COVID-19 booster. The doctor, not revealing his true identity, then arranged to administer the shot during a Jan. 22 home visit, Northumbria Police said.

Investigators said Kwan put fake license plates on his car before driving to a hotel the morning of the visit. Disguised with a wig, fake facial hair and a medical mask, Kwan injected the elderly man with a toxin that caused a “rare, life-threatening flesh-eating disease,” court officials said, according to BBC  news reports.

The man required several operations, including skin grafts, to repair damage from the injection and “life-changing injuries,” police said.

“He has been through a horrendous ordeal and his life has been changed forever,” Northumbria Police Detective Chief Inspector Jason Henry said in an official statement. “He has shown incredible strength throughout the investigation and we will continue to support him in any way that we can.”

Officers arrested Kwan at his home two days later. They found on his computer what police called a “poisoner’s handbook,” a guide for murder investigations and several files with information about how to kill a person with poison and not get caught, according to the news release.

They also said Kwan installed spying software on his mother’s computer to monitor the woman and her partner for more than a year.

A trial in the case began last week but Kwan changed his plea on Monday, prompting jurors to return a guilty verdict.

Happy House Surgery, where Kwan worked before his arrest, sent a message informing patients of the charges in early February.

“As this is an ongoing police matter, we are unable to comment any further,” the letter stated. “However, we would like to reassure patients that the practice is open as normal and patients should feel confident in continuing to access services at Happy House Surgery.”