An NHS doctor killed his partner’s father and almost caused the deaths of her and her mother by giving them a poison that is popular among murderers, a London court has ruled.

He poisoned the trio with thallium then tried to cover his tracks, did not deploy his medical knowledge to help his partner and lied about what happened, the judge found.

The ruling has been handed down by Mr Justice Williams in the family court at the end of a case between the doctor and his now former partner over the arrangements for the care of their child. The case is highly unusual legally because the medic has been found to have committed the killing in a civil court, on the balance of probabilities of the evidence, rather than in a criminal court, in which a jury has to be sure beyond all reasonable doubt.

“It is truly horrifying that any doctor who has signed the Hippocratic oath would be responsible for killing someone else and attempting to kill other people,” said Simon Bruce, a senior family law specialist solicitor who was part of the woman’s legal team.”

While most cases that come before the family court involve injuries, harassment and domestic abuse, Bruce added, “the killing of the child’s grandfather, and the attempted killing of the mother and grandmother, are off the scale of unusualness”.

The Guardian is unable to disclose the doctor’s name because in the judgment the judge ordered that his identity, the mother of his child and their son had to remain anonymous.

However, Williams said in his judgment that the poisoner was a Bulgarian-born doctor who had moved to the UK in the late 2000s. The medic had a relationship with the woman, a bookkeeper and fellow Bulgarian who had been granted British citizenship, and their child was born in 2010.

The court found that he added thallium to a pot of coffee when he, his then partner, their then one-year-old child and her parents were on holiday in Bulgaria in September 2012. His partner and her parents drank from the pot while the doctor had instant coffee instead. The child’s grandfather died two days later. The two women became seriously ill but survived. The doctor and his partner separated soon afterwards.

The Bulgarian police have made inquiries into the poisoning and interviewed the medic, and the Metropolitan police have also been notified. But he has not been charged with any criminal offence.

“The circumstances around my father’s death and the thought of how he had suffered from the effects of the poison in his last hours will haunt me forever. No one could possible imagine the excruciating pain my mother and I felt as a result of the poisoning in the first six weeks after we had been poisoned,” the woman told the Guardian.

“Every day I suffer tremendously bearing the thought that I brought a murderer to my parents’ house and that my dad died as a direct result of this. What I have been through is a nightmare.”

The doctor began legal action in 2018 in an attempt to gain access to his son after he and his partner separated. She argued that he should not have contact because he had poisoned her parents and herself.

The Centers for Disease Control and Prevention in the US say that thallium, a heavy metal, “is tasteless and odourless and has been used by murderers as a difficult to detect poison”.

After a five-day hearing last year, Williams ruled in favour of the mother. He found that on the balance of probabilities her claim that he had poisoned her and her parents was correct.

During the hearing the doctor’s former partner told how he insisted they leave Bulgaria for Britain when the debilitating effects of the poisoning were starting to appear. She said that once back in the UK, he did little to help her get medical help, despite her hair falling out and her suffering sudden weight loss. It was later determined that the woman, her mother and her father had all been poisoned.

In what the judge said was a ruse to evade detection for what he had done, the doctor told fellow medics seeking to identify the source of her sickness that she may be suffering from Guillain-Barré syndrome, a rare and serious nerve condition.

In his judgment Williams several times noted that the doctor had told lies in the different accounts he had given of events surrounding the poisoning. His evidence during the hearing was “rather fractured” and at time “felt like a construct” and he was “insincere” when answering questions, the judge noted. “Overall, I found him to be a quite unsatisfactory witness,” he added.

He also noted: “The father as a qualified medical doctor would have had the intellectual capacity to determine the amounts of thallium that would be required to deliver a sufficient dose to kill an individual without delivering such a significant dose as to make it obvious to the consumer of the drink that it had been contaminated.”

“At a time when the mother was frantic and had come to believe that she was poisoned, his failure to do anything of substance on the medical front is almost inexplicable save by prior knowledge of the causes,” he added.

The doctor is not currently working. The Guardian sought his response to the court judgment. A statement issued by Delphine Philip Law, his solicitors, said: “He vehemently denies the mother’s allegations that he poisoned her or her parents. He stated that the court took inadequate account of the fact that there was no direct evidence to collaborate [his former partner’s] allegations.”