Northern Ireland police received an extra 48 hours on Friday to question Sinn Fein leader Gerry Adams about an Irish Republican Army killing of a Belfast widow.
The development has infuriated his Irish nationalist party and raised questions about the stability of the province's Catholic-Protestant government.
The Police Service of Northern Ireland confirmed in a statement its detectives received permission at a closed-door hearing with a judge to detain Adams for up to two more days.
Had the request been refused, Adams would have had to be charged or released by Friday night, two days after his arrest as a suspect in the 1972 abduction, slaying and secret burial of Jean McConville, a Belfast mother of 10.
The new deadline is Sunday night, although this too could be extended with judicial permission.
Earlier on Friday, the senior Sinn Fein politician in Northern Ireland's unity government had demanded the immediate release of Adams, saying police were seeking to extend his interrogation over the killing of McConville.
Deputy First Minister Martin McGuinness condemned the police's continued questioning of Adams.
McGuinness accused "a cabal" of officers within the Police Service of Northern Ireland of pursuing "a negative and destructive agenda to both the peace process and to Sinn Fein."
Police arrested Adams on Wednesday night as a suspect in the IRA abduction, killing and secret burial of McConville, a mother of ten.
Under British "anti-terror" laws, suspects must be charged or released within 48 hours, unless police receive a judicial extension.
Britain's 2006 terrorism law permits such extensions up to a maximum 28 days.
Typically in Northern Ireland, terror suspects' detentions are extended by one to five days.
Northern Ireland police do not announce in advance when they seek such extensions, but do confirm them once they have been approved.