Ben Roberts-Smith has labelled as “disgraceful” claims by the legal team representing three newspapers at his defamation trial that he killed a teenage boy in Afghanistan.

Mr Roberts-Smith, 42, is suing The Sydney Morning Herald, The Age and The Canberra Times at a Federal Court trial in Sydney over allegedly defamatory media reports from 2018 claiming he was involved in war crimes, murders and bullying in Afghanistan with the SAS.

He denies the claims while the news outlets defend them on the basis of truth.

On Monday, Mr Roberts-Smith’s barrister Bruce McClintock SC asked his client, in re-examination, about respondents’ assertions earlier at the trial that he killed a teenage boy while in Afghanistan.

The court heard that Mr Roberts-Smith at the battle of Tizak in 2010 shot dead a “young man” who was armed with a machine gun.

“From that moment I have lived with that every day and I struggle with it,” Mr Roberts-Smith told the court.

“To have the respondents lecture to me about shooting and murdering a teenage boy it’s just shameful, it’s shameful because not only did it not happen but they know it didn’t happen, so they change their story again to fit what they need it to fit.

“They know it’s false, they know it, and they still say it … it’s disgraceful.”

He told the court he was also “shocked” to hear suggestions from the respondents’ legal team that he ordered another SAS soldier to shoot an old man on an April 2009 mission on Taliban compound Whiskey 108, located in Uruzgan province.

“I was upset that again my professionalism was being targeted by these individuals and such an outrageous claim was being made,” he said.

“It’s very difficult to understand that they can’t just accept they’ve got it wrong and it’s false.”

Earlier, evidence was heard from former Liberal politician Brendan Nelson who told trial that stories critical of the war hero seemed intent on “bringing him down”.

Mr Nelson, a witness for the applicant, said he knew Mr Roberts-Smith as an adored public figure and soldier who was saluted by higher-ranking members of the military due to his Victoria Cross.

He told the court the former SAS soldier was “clearly devastated” by the allegedly defamatory articles.

The former Australian War Memorial director said following the reports Mr Roberts-Smith appeared less confident, less outgoing, and more anxious.

“He’s apprehensive about what people think about him,” Dr Nelson said.

The trial continues before Justice Anthony Besanko.

-AAP