The Victoria Cross recipient sued Nine newspapers and journalists Nick McKenzie and Chris Masters over their reports in 2018 which claimed he had committed war crimes.
He suffered a bruising loss in 2023 when Justice Anthony Besanko found the claims the former soldier was responsible for the murder of four unarmed civilians were substantially true.
The former SAS corporal launched an appeal against the findings in 2024, which was dismissed by the full Federal Court on Friday after more than a year of consideration.
Within an hour of the decision being handed down, Roberts-Smith indicated he would immediately seek to challenge the decision in the High Court.
"I continue to maintain my innocence and deny these egregious, spiteful allegations," he said in a statement.
"Sunlight is said to be the best disinfectant, and I believe one day soon the truth will prevail."
The first hurdle for the former special forces soldier will be obtaining special leave to appeal to the High Court, defamation barrister Roger Rasmussen told AAP.
"Special leave is always extremely difficult (to obtain) in any case … because it's not an appeal as such," he said, adding that he has no knowledge of the case.
"You've got to show some special reason for why the High Court should be entertaining your application."
The reasons may be the interest of justice or a legal point of law, which in Roberts-Smith's case would relate to the standard of proof.
A silk told AAP there was nearly no chance Roberts-Smith could succeed in arguing his appeal was in the interest of justice, because four judges had reached the same conclusion.
He said the application for special leave would require creativity and the soldier's legal team would need to "change tack".
"It's not an impossible task," he concluded.
Another senior legal figure suggested Roberts-Smith's team might push for an appeal based on Justice Nye Perram's refusal to admit evidence from Roberts-Smith's ex-lover on an interlocutory application.
Justice Perram is due to hand down the reasons for his decision in due course.
"There was a case last week where the High Court overturned a decision because of similar (appeal grounds)," the legal figure said.
She said she was left "uneasy" with the outcome because a court had ruled it was satisfied war crimes occurred when Roberts-Smith has not been charged by a prosecuting authority.
Mr Rasmussen said the attention given to the case could alone be sufficient to obtain special leave to appeal.
"The High Court doesn't usually worry too much about that, but they might consider it's of sufficient importance … because they are pretty serious allegations," he noted.
If Roberts-Smith obtains special leave, Mr Rasmussen said it could take months for the High Court to vet the application and decide whether to extend an invitation to appeal.
He was confident the former SAS corporal's lawyers were already preparing their submissions.
The appeal decision handed down by Justices Perram, Anna Katzmann and Geoffrey Kennett is yet to be published.
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