Richard Di Natale has been forced to apologise to Kevin Rudd, after the Greens leader called the former prime minister a "sociopath" on live television.

In a letter to Mr Rudd, Senator Di Natale said he acknowledged that using the term was a "serious slur".

"I unreservedly retract the suggestion that you are a sociopath," Senator Di Natale wrote in the letter dated February 7, 2019.

Greens leader Richard Di Natale has apologised to former prime minister Kevin Rudd. Credit:AAP

"I realise that this is a serious slur on your reputation as it is a clinical condition of a mental illness."