Jamie Bryson last night threatened legal action to trace the source of leaked 'private' messages between him and Sinn Féin figures in the run-up to giving evidence to the Stormont finance committee last September.
The Co Down loyalist denied that any of the information he has placed in the public domain regarding the £1.2 billion Nama property deal came from Sinn Féin.
But he said his solicitor would be asking for an investigation into how the supposedly private correspondence about his evidence was leaked into the public domain.
Despite messages that show Daithí McKay and party colleague Thomas O'Hara advising Mr Bryson on tactics prior to his committee appearance, he said claims of coaching were "absolute nonsense".
[ Who is Thomas O'Hara?Opens in new window ]
Mr McKay resigned as an MLA yesterday after The Irish News revealed he had communicated with the loyalist by private message before putting him in touch with a party colleague to act as a 'back channel' advising him how to get his evidence into open session.
Mr Bryson was told to refer to former DUP leader Peter Robinson as 'Person A' throughout his committee evidence before finally naming the then First Minister at the conclusion.
Speaking to the Irish News yesterday, Mr Bryson said he did not believe that the former chair of the finance committee was on a 'solo run' when they exchanged messages.
"I have instructed legal representatives to initiate a number of complaints in an attempt to ascertain how these communications were obtained. I believe it was illegally obtained," said Mr Bryson.
"This morning I refused to discuss the content of the communications due to the fact that I did not want to place Daithí McKay in an untenable position.
"I was in no doubt, and remain in no doubt, that Mr McKay was not acting on a solo run.
"I suspect it is for that reason that Mr McKay was forced to resign probably more swiftly that any politician in any political scandal in living memory."
