SINN Féin has said DUP minister Edwin Poots must further explain his decision to approach the attorney general about his son facing a driving charge.
It warned that no explanation risked leaving the impression that Mr Poots was "attempting to abuse his position as an elected public representative".
Mr Poots has strongly defended writing to the attorney general over the case relating to ex-DUP councillor Luke Poots.
The Lagan Valley MLA said he was warning of alleged press leaks which could "damage the independence of due process".
His full letter to Stormont's chief legal adviser was revealed by The Irish News on Monday.
It was released by the attorney general's office following calls for full disclosure after its existence emerged this month.
In the letter, Mr Poots disclosed that he was a passenger in the vehicle, and accused police or prosecutors of leaking the incident to the media to cause "maximum embarrassment".
He also appeared to suggest that he and his son may have feared a security threat.
Mr Poots said they wanted to be treated the same as anyone else, and asked the attorney general to ensure "fairness is applied".
Luke Poots, who works in his father's constituency office, was accused of driving dangerously on Grosvenor Road in Belfast in December 2017.
The 31-year-old denied the charge but was convicted of a lesser offence of driving without due care and attention. He was fined £500 and given five penalty points last month.
Edwin Poots wrote in 2018 to then Attorney General John Larkin, who forwarded the correspondence to the Director of Public Prosecutions and asked him to examine the matter.
The Irish News had revealed Luke Poots's driving charge the day before he was due in court. At the time, court lists were available to the media seven days in advance.
However, Mr Poots has repeated his allegation that the driving charge was "leaked" to the press.
A Sinn Féin spokesman said: "The public must be afforded an explanation from DUP minister Edwin Poots of why he thought it was necessary or appropriate to write to the attorney general on a driving offence involving his son and the criminal justice system.
"Without such explanation the public are surely left with the impression that Edwin Poots, who was apparently a passenger in the car, was attempting to abuse his position as an elected public representative who has a responsibility to uphold the rule of law."
The Public Prosecution Service (PPS) has said its director reviewed the case and concluded the "test for prosecution had been properly applied".
It said "no concerns around the conduct of the investigation or prosecution were identified".
The PSNI said anyone with a complaint can contact the Police Ombudsman for independent investigation.
