Sir Bob Jones is demanding an apology and retraction after a University of Waikato professor called him "racist" on social media.
Associate professor Dr Leonie Pihama, the director of Te Kotahi Research Institute at the university, was served with a letter by Jones' lawyer John Langford last Friday.
The letter referred to a tweet sent by Pihama the day before, in which she called Jones "racist".
It follows the news that Jones filed defamation proceedings against a Māori filmmaker who formed a petition to revoke his knighthood.
"The term 'racist' is clearly defamatory, by any reasonable standard. It was simply used as a label in this case and cannot be defended as part of some wider debate, or discussion about race issues," Langford wrote in the legal letter.
"Sir Robert requires you to immediately withdraw, and retract, your use of this label, and apologise to him, via the same media in which it was published."
Langford concluded Jones' next steps would be determined by Pihama's approach to the letter.
This morning, Langford confirmed the authenticity of the letter - which has been circulated on social media during the past 12 hours.
Hundreds of tweets by other social media users have echoed a similar tone to Pihama's in response.
"My letter was respectful in tone, and a respectful response would have been appreciated," Langford told the Herald.
Pihama's tweet was accompanied by a link to the petition to revoke Jones' knighthood.
She told the Herald she has not responded to Langford's letter.
"However I received a follow up email on June 11. I have chosen not to respond as I do not respond to threats," she said.
"If Mr Langford is going to file and the formal documentation is delivered to me I will then engage, until such time the letter is, in my view, an example of using the legal system as a threat to silence and I will not be bullied in such a manner."
Pihama's tweet came after the Herald broke the news that Jones had filed defamation papers in the High Court at Wellington against filmmaker Renae Maihi.
More than 68,000 people signed Maihi's petition, which was delivered to Parliament in March, in response to Jones' February 2 column in the National Business Review calling for a Māori "Gratitude Day" instead of Waitangi Day.
The petition was presented to Labour MP Kiritapu Allan with the support of Waikato University Professor Pou Temara.
The petition read: "In signing this petition we urge you, our Prime Minister the Rt. Hon. Jacinda Ardern, to take his knighthood away from him. It is in your power. Set a precedent for the country and a message that this will not be tolerated..."
Jones' knighthood is for services to the business community.
Jones' NBR column started with the opening line "time for a troll" before he said: "I have in mind a public holiday where Māori bring us breakfast in bed or weed our gardens, wash and polish our cars and so on, out of gratitude for existing.
"And if any Māori tries arguing that he/she didn't have a slight infection of Irish blood or whatever, they might be the better for it, the answer is no, sunshine."
NBR removed the column, which Jones said was satire, from its website "due to inappropriate content". Jones has since stopped writing his column for NBR.
A Press Council complaint by Mel Whaanga, who said Jones' comments were "racist", was also dismissed in April.
The Human Rights Commission also released a statement praising the removal the column from NBR's website.
"Sir Bob Jones and those outlets who choose to publish this kind of rhetoric need to be prepared for the public backlash and condemnation they provoke and deserve."