Former assistant chief constable Alan McQuillan is to sue the SDLP and the party leader Mark Durkan, his lawyer has confirmed.
Alan McQuillan will sue the SDLP and its leader Mark Durkan
Paul Tweed said he will be issuing a libel writ on Thursday.
The legal action follows comments made in a SDLP newspaper advertisement in January.
Mr Tweed said his client wanted "in the absence of a satisfactory apology from Mark Durkan, to seek total vindication of his reputation in the courts".
The newspaper advert related to the SDLP's role in the selection process which led to the appointment of Sir Hugh Orde as PSNI chief constable.
In a statement responding to news of the legal action, the SDLP said: "We have never said in any shape or form that he would have been an improper appointment (as chief constable).
"We have been happy to give that assurance before and reiterate it now but we cannot apologise for what we have not said."
Mr McQuillan, who is currently head of the Assets Recovery Agency, had also applied for the job.