DUP Assembly Member Jimmy Spratt has said sorry for describing those who oppose the conflict resolution centre at the Maze as "nutters" – after initially denying making the comments.
At first Mr Spratt had denied using the word in relation to opponents of the development at the site of the former Troubles-era jail – and even said he would be seeking legal advice over the matter.
However, the verbatim record of the Committee of the Office of First Minister and Deputy First Minister meeting revealed that Mr Spratt did use the term.
Mr Nesbitt said that Mr Spratt "knows what he has to do now" after the official Stormont report – known as Hansard –was published.
"In OFMDFM committee on Wednesday, Jimmy Spratt said, 'We will see whether it is on the record in the Hansard report', in relation to words he uttered during comments about opponents of the Maze project," said the UUP leader.
"Hansard has now been published and is very clear. During a comment from DUP MLA, Brenda Hale saying, 'There is no local opposition to the Maze or the buildings within it', Mr Spratt is quoted as saying, 'Except the nutters'. Mr Spratt now knows what he has to do. The ball is in his court."
Last night Mr Spratt apologised.
"My comments were in no way related to any of the genuine people who have expressed concerns about the Maze project," he said.
"I attempted to make it clear at the time that my remark was not directed at any groups or individual. However, I apologise sincerely for any hurt caused to those who believed my comments were directed at them."
Mr Spratt said he had been subject to a great deal of personal abuse, often from people who have online anonymity.
"As someone who served for many years in the Royal Ulster Constabulary and lost many close friends to terrorists, I find such comments particularly hurtful."
First Minister Peter Robinson had been telling MLAs at the committee he was convinced there would be "nothing in the centre to offend anyone".
The DUP and Sinn Fein have backed the controversial prison site development but Mr Nesbitt fears that it could end up becoming nothing more than a shrine to IRA terrorism.