Just hours after Cummins delivered an extraordinary 70-minute press conference denying any prior knowledge of the 2018 sandpaper plot, Warner hit back on Friday.
"Pat Cummins was very well prepared this morning and I still stick by Uzzie and what he has written in that book," Warner on Kayo Cricket.
"It is the truth.
"I actually feel for him (Khawaja) at the moment. I actually feel that the captain today threw Uzzy under the bus and I'm very disappointed by that."
Warner's comments came after he had earlier this week labelled Khawaja's claims in his book as "100 per cent accurate".
Included in Khawaja's bombshell revelations were Australia had used sandpaper in the Test prior to Cameron Bancroft being caught in Cape Town in 2018.
Khawaja also implied the entire team knew of a ball-tampering plot and Warner, Bancroft and Steve Smith were victims of a rushed review that found only the trio had knowledge.
Cummins made a point to agree the trio were dealt with harshly during his extraordinary press conference on Friday, but said Khawaja's claims were "complete nonsense" and suggested they were "probably defamatory".
"When he says we all knew, if by that he means that we knew sandpaper was going out in the field, he's 100 per cent wrong," Cummins told a packed press conference in Sydney.
"I've said that consistently for the last eight years. I did not know that was happening.
"But Usman actually says things that are actually far more serious than that. He says that I and others were architects of a scheme to ball tamper with sandpaper.
"That's complete and utter nonsense."
Cummins said he did not remember any conversations around the team taking collective responsibility, which according to Khawaja in his book said he had pushed for.
The 33-year-old also revealed he had exchanged some messages with Khawaja over recent days and planned to speak to him at some point.
But the pair's relationship is now considerably rocky.
"Usman's a friend, but friends aren't casual with each other's lives and reputations," Cummins said.
"In his book, Uzzy's been casual with my life and the lives of many others, and I think Australian cricket.
"I support his right to tell a story. I'm really glad that he feels like he can. But we have to be accountable for the things that we say and do."
Cummins said it was up to Cricket Australia whether they would reopen the investigation into the matter, something the organisation has already ruled out.
But he made clear he wanted to move on from the scandal, declaring his willingness to take every question on Friday as it would be the last time he speaks about it.
"I did get advised this week that some of the things that were said are probably defamatory," Cummins said.
"But I won't be doing that. Like, you've got to move on. It's been eight years.
"We're moving on. You've got to draw a line at some point."
Now the Australia Test captain, Cummins pointed out at the time of the scandal in South Africa, he was a 24-year-old just 13 Tests into his career.
Cummins said he had also not yet spoken to Warner since his backing of Khawaja's claims, and could not explain why their position was so different to his.
"That sounds like a pretty good question for those two," he said.
"Again, there's quite a few different accounts here, so they've got two accounts.
"There's 11 players in that room, so that means there's nine other guys who I think it's appropriate you give an equal amount of weight into their story as well."
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