One of Australia's most decorated soldiers on Friday lost a legal bid to overturn bombshell court findings that implicated him in war crimes while serving in Afghanistan.

Former SAS commando Ben Roberts-Smith has been fighting to repair his tattered reputation since 2018, when newspapers unearthed allegations he took part in the murder of unarmed Afghan prisoners.

His multi-million dollar bid to sue three Australian newspapers for defamation failed in 2023, with a judge ruling the bulk of the journalists' claims were "substantially true".

The 46-year-old suffered another setback on Friday, when Australia's Federal Court dismissed his appeal.

Justice Nye Perram withheld the reasons for the decision, saying there were national security implications the government must consider before they are released.

A published summary said there was sufficient evidence to support findings that Roberts-Smith had "murdered four Afghan men".

Perth-born Roberts-Smith had been Australia's most famous and distinguished living soldier.

He won the Victoria Cross – Australia's highest military honour – for "conspicuous gallantry" in Afghanistan while on the hunt for a senior Taliban commander.

The Age, The Sydney Morning Herald and The Canberra Times shredded this reputation with a series of reports in 2018.

They reported Roberts-Smith had kicked an unarmed Afghan civilian off a cliff and ordered subordinates to shoot him.

He was also said to have taken part in the machine-gunning of a man with a prosthetic leg, which was later brought back to an army bar and used as a drinking vessel.

The 2023 court ruling ultimately implicated Roberts-Smith in the murder of four unarmed Afghan prisoners.

Civil court matters such as defamation have a lower standard of proof than criminal trials.

Roberts-Smith has not faced criminal charges.

Australia deployed 39,000 troops to Afghanistan over two decades as part of US and NATO-led operations against the Taliban and other militant groups.

A 2020 military investigation found special forces personnel "unlawfully killed" 39 Afghan civilians and prisoners, revealing allegations of summary executions, body count competitions and torture by Australian forces. (AFP)