Greens leader Richard Di Natale has apologised to Kevin Rudd after calling the former prime minister a sociopath on live television.
Senator Di Natale acknowledged the term was a "serious slur" and a clinical condition of mental illness.
"I unreservedly retract the suggestion that you are a sociopath," he wrote last week in a letter to Mr Rudd and cited by The Sydney Morning Herald on Tuesday.
"I realise that this is a serious slur on your reputation as it is a clinical condition of a mental illness."
Senator Di Natale made the slur against Mr Rudd during a Q&A program on ABC television in December.