Australia's top court has thrown out a last-ditch attempt by the country's most decorated soldier, Ben Roberts-Smith, to overturn a landmark defamation ruling.

Thursday's ruling ends Mr Roberts-Smith's long legal battle with newspapers he had sued over reports of serious misconduct while he was deployed to Afghanistan.

A judge in 2023 ruled that news articles alleging the Victoria Cross recipient had murdered four unarmed Afghans were true, but he argued the judge made legal errors.

It was the first time in history any court assessed claims of war crimes by Australian forces. Mr Roberts-Smith lost an appeal against that judgement and was ordered to pay the newspapers' legal costs.

Mr Roberts-Smith, who left the defence force in 2013, has not been charged over any of the claims in a criminal court where there is a higher burden of proof.