The decision was made by three judges in Victoria’s Court of Appeal in Australia, which dismissed an attempt by Indian activist and elder Robbie Thorpe to bring a private accusation against the monarch.
King Charles is the head of state of Australia as well as the king of Great Britain.
The ruling leaves Mr Thorpe with a remaining home sentence, and is seeking permission to appeal to the High Court of Australia.
ABC News reports that the Supreme Court will only allow a small proportion of the requests for appeal.
Mr Thorpe, a 68-year-old elder of the Krautatungalung people, has been trying to start legal proceedings against the king in Melbourne since 2023.
His case alleged that the monarch, along with Australian government institutions, was committing genocide through systems that disadvantaged Indigenous Australians.
Following the latest ruling, Mr Thorpe indicated that he intended to pursue the matter internationally.
He said: “The International Criminal Court will expose Australia for what it really is – a lying, racist, illegal occupation committing genocide.”
Monday’s decision marks Mr Thorpe’s third legal defeat since the start of his campaign to bring the case to court.
According to ABC News and AP, Indigenous Australians make up about four percent of the country’s population and experience poorer health outcomes, lower life expectancy and higher rates of unemployment and incarceration than other Australians, according to official statistics.
Legal experts quoted by the news organizations said the British monarch enjoys extensive immunity from criminal and civil liability in Britain.
Constitutional lawyer Anne Twomey, of Sydney University, said this protection was likely to be inherited from Australia, although she noted that she was not aware of the issue being tested before in an Australian court.
The reports also note that while the International Criminal Court does not recognize immunity for heads of state, individuals cannot initiate prosecutions before the court themselves.
International law expert Don Rothwell of the Australian National University said individuals could instead submit evidence for prosecutors to consider.
He said: “An individual cannot initiate a prosecution. Once they have exhausted all their options under Australian law, the best they can do is seek to provide evidence that they believe constitutes the crime of genocide.”
“It is then up to the International Criminal Court to determine whether to try to prosecute.”