A senior Assembly official has been accused of abusing his position after refusing to act on a complaint from a DUP MP.
Sammy Wilson insisted Commissioner for Standards Douglas Bain should "do his job" without seeking to impose conditions.
The East Antrim MP previously blasted Mr Bain as a "wee wimp" after he found Mr Wilson guilty of breaching rules by calling Jim Allister a "thug".
But now the public row is escalating into a full-blown dispute involving Assembly procedures, with the prospect that another external commissioner may have to be brought in.
Former minister Mr Wilson warned of a "ham-fisted vendetta" against him after Mr Bain asked for assurances the senior DUP man had confidence in him.
The MP said: "I expect him to do his duty. He is well-paid for his services and I would have thought that he would do his job without seeking to put down conditions. Why should what I have said in the past, which I do not withdraw, have anything to do with it?"
Mr Bain stressed: "Like any judge, the commissioner must not act in any complaint in which he has or could be perceived to have an interest or to be biased."
And he argued Mr Wilson's suggestion of a vendetta had confirmed his initial view "that I could not properly deal with his complaint".
Mr Bain declined to undertake a complaint from Mr Wilson against Sinn Fein MLA Mairtin O Muilleoir that was linked to the Assembly's probe into the Nama scandal.
After the request, the Assembly Commissioner for Standards demanded an unequivocal statement from Mr Wilson that he wanted Mr Bain to deal with the issue. It came after Mr Bain sent Mr Wilson a solicitor's letter following his "wee wimp" attack.
Mr Bain declined, however, to comment on any legal action he has pursued since.
"In view of what had passed between myself and Mr Wilson, I considered that there was a significant risk that any action I took in relation to his complaint might be perceived - either by him or by the Member complained about - as being biased," Mr Bain said.
"In an attempt to address that matter, I wrote to Mr Wilson explaining that unless he confirmed to me in writing that he was content that I should deal with his complaint, I would seek the appointment of an acting commissioner to deal with it."
Mr Wilson said he had read the letter with "some incredulity" and replied: "I would point out you are a public servant whose job is to investigate without conditions.
"Secondly I would point out I brought the complaint to you with the expectation you would carry it out since that is what you are paid to do."
The former Finance and Environment Minister asked Mr Bain to provide examples if his demand was a regular requirement.
He also asked whether it was "some ham-fisted attempt to carry out a vendetta against someone who had the temerity to stand up to you".
Mr Wilson refused to apologise to Mr Bain last year, insisting it would be hypocritical because any such apology would not be sincere.
He had called TUV leader Mr Allister a thug during a meeting of the Assembly's social development committee, which was examining allegations broadcast in a BBC Spotlight investigation of political interference in the running of the Housing Executive (HE).
Jenny Palmer, a former DUP councillor who is now in the UUP, said she was put under pressure by DUP special adviser Stephen Brimstone to change her vote on the HE board in relation to contractor Red Sky.
The committee backed Mrs Palmer's version of events and accused Mr Brimstone and then minister Nelson McCausland of "inappropriate actions".
While Mr Allister questioned Mr Brimstone, Mr Wilson referred to him as a "thug" and SDLP member Dolores Kelly referred the matter to Mr Bain.
He then ruled the term contravened the respect principle within the MLAs' code of conduct, concluding it was an "unreasonable and excessive personal attack" on Mr Allister.
Mr Wilson hit back, however: "I'm not sorry for what I said. I believe that in the context in which I said it, it was correct."
The Assembly Committee on Standards and Privileges supported Mr Bain's finding, but it rejected his assertion that Mr Wilson's behaviour had brought the Assembly into disrepute.
June 25, 2015
Sammy Wilson refers in the media to Douglas Bain as a "wee wimp" and "faceless Commissioner". Mr Bain said he regarded the remarks as offensive and defamatory and his solicitor sent Mr Wilson a "pre-action letter in the usual terms". He declined to comment on what action he has taken since.
Following his appearance before the Assembly committee examining the NAMA allegations, Mr Wilson writes to Mr Bain asking him to undertake an investigation into south Belfast Sinn Fein MLA Mairtin O Muilleoir.
"I know from past experience you are fastidious in ensuring the rules of the Assembly are followed by members," Mr Wilson says.
There is an acknowledgement from Mr Bain and then a reply asking for an unequivocal statement of confidence from Mr Wilson in the Commissioner.
"Before I take any action I wish to be sure that you are content in light of your comments about me.
"While I would not let these matters influence, I do not consider it would be appropriate in the absence of an unequivocal statement from you that you have no objection to me dealing with your complaint."
Mr Bain says if he has not received a letter by January 7 he "shall proceed on the basis that you are not content, and will seek an independent commissioner".
January 4 2016
Mr Wilson replies: "It was with some incredulity I read your response...especially your demand for certain conditions to be met before you proceed. I would point out you are a public servant whose job is to investigate without conditions.
"Secondly I would point out I brought the complaint to you with the expectation you would carry it out since that it was what you are paid to do."
The MP asks if Mr Bain makes such demands "a regular requirement and if so "could you provide some examples," and then asks whether it is "some ham-fisted attempt to carry out a vendetta against someone who had the temerity to stand up to you?"
Mr Wilson again writes to Mr Bain, saying despite a second letter being sent, he still has no confirmation that Mr Bain intends or has begun to carry out any investigation.
He suggests if this is the case it would amount to a "very serious breach of your duty as a public official.
"I would appreciate within seven days sending me a response as to your intention (because) there is some journalistic interest."
Mr Bain responds saying he has noted the contents of the letter and the issue is now in the hands of the committee for standards in the Assembly.
He said in light of Mr Wilson suggestion "that I might be pursuing a vendetta against him... was clear that I could not properly act in relation to his complaint and advised the Committee on Standards and Privileges of my decision".
A statement to the Belfast Telegraph added: "I did not in my letter of 17 December 2015 seek an apology from Mr Wilson for his defamatory comments. The only requests for such an apology have been in the correspondence sent to him by my solicitors."
Mr Wilson replies once more, asking Mr Bain, since this matter is in hands of the committee "could you let me know what investigations were carried out by yourself (and) who was interviewed?"