•Thomas Kwan injected Patrick O’Hara with a chemical weapon which left him with a flesh-eating disease

His victim, Patrick O’Hara, 72, was left with a rare flesh-eating disease as a result of the injection in his arm.

Thomas Kwan, 53, was on trial at Newcastle Crown Court and had initially denied attempted murder, but changed his plea after he heard the prosecution open the case against him.

Police initially thought the married father-of-one used the chemical weapon ricin to try to kill Mr O’Hara at his mother’s home in St Thomas Street, Newcastle, on 22 January but an expert believed a pesticide was more likely.

The Hong Kong-born doctor had developed an “encyclopaedic knowledge” of poisons, the court heard, and he studied how to get away with murder, police discovered from analysis of his home computers.

His actions sparked a major emergency services operation after lethal chemicals were discovered by police at his detached garage at his home in Ingleby Barwick, Teeside.

The Sunderland-based GP had already pleaded guilty to administering a noxious substance, claiming he meant to cause no more than mild pain.

The court heard that Kwan was “money-obsessed”, and that Mr O’Hara posed a “potential impediment” after his mother said in her will that he could remain at her property if she were to die before him.

Opening the case on Thursday, Peter Makepeace KC, prosecuting, said: “Mr Thomas Kwan, the defendant in the case, was in January of this year a respected and experienced medical doctor in general practice with a GP’s surgery based in Sunderland.

“From November 2023 at the latest, and probably long before then, he devised an intricate plan to kill his mother’s long-term partner, a man called Patrick O’Hara.

“On any view, that man had done absolutely nothing to offend Mr Kwan in any way whatsoever.”

Kwan forged NHS documentation to set up a home visit with Mr O’Hara, after contacting him by claiming to be a community nurse called Raj Patel and offering him a home visit.

Mr Makepeace said: “As, I suspect, would any of us, Mr O’Hara fell for it hook, line and sinker, he had not the slightest suspicion that this was anything other than a genuine NHS community care initiative which he warmly welcomed and was grateful for.”

After checking in to a city centre hotel using a fake name, he travelled to his mother’s house wearing a disguise and using false number plates.

a long coat, flat cap, surgical gloves and wearing a medical mask and tinted glasses, and carried out a 45-minute examination on Mr O’Hara, and even checked his unsuspecting mother’s blood pressure when she asked.

Wearing a long coat, a medical mask and tinted glasses, Kwan, in what the court heard was broken English with an Asian accent, told Mr O’Hara he needed a Covid booster, even though he had only had one three months ago.

Mr O’Hara shouted in pain when it was administered and Kwan quickly packed his equipment and left, reassuring his victim that a reaction was not uncommon.

The pain continued and Mr O’Hara began to suspect something had gone badly wrong.

The next day his arm had blistered and was seriously discoloured and medics at hospital were baffled.

He had developed the flesh-eating disease necrotising fasciitis and needed to have part of his arm cut away to stop it spreading, and spent several weeks in intensive care.

The fake nurse’s movements were traced using CCTV and police were able to identify Kwan as a suspect.

Searches of his home in the executive estate where he lived revealed an array of chemicals such as arsenic and liquid mercury as well as castor beans which can be used to make the chemical weapon ricin.

Police found a recipe for ricin on his computer but Ministry of Defence poisons expert Professor Steven Emmett, although still not sure which poison was used, thought iodomethane which is commonly used in pesticides, was more likely.

In the years before the attack, Kwan had led a strained relationship with his mother, Jenny Leung, over her decision to include Mr O’Hara in her will, which led to police being called in November 2022 after Kwan burst into her home.

He went on to install spyware on his mother’s laptop so that he could secretly monitor her finances.

Mrs Justice Lambert said she will sentence Kwan next Thursday once the issue of his dangerousness has been considered by the Probation Service.