Hailemariam Desalegn, who assumed the prime minister's office upon the death of Meles Zenawi Monday, will finish Zenawi's five-year term.
Meles, 57, died late on Monday in a Brussels hospital after a long fight against illness, leaving a gaping hole in Ethiopian politics and depriving Western powers of a trusty ally in the fight against Islamist militants in the Horn of Africa.
"He will be the prime minister until 2015. He is to be sworn in and he is to finish the five-year term of government and that is indisputable," Bereket Simon told Reuters by telephone.
Bereket said parliament would be summoned within the next two days and Hailemariam would be sworn in as prime minister.
Crowds of mourners, many holding candles, gathered to witness Meles' casket upon its arrival in the capital, Addis Ababa, late on Tuesday. His body is now lying in his private residence as preparations are made for a state funeral.
Bereket confirmed the ruling Ethiopian People's Revolutionary Democratic Front party would hold a party congress at an undisclosed date to nominate a leader but said there was no scenario under which Hailemariam would be ousted as premier.
Speculation had been rife that a potentially divisive race for the top post would follow Meles' death. Asked whether all coalition members of the party had approved Hailemariam's selection, Bereket said: "They have no problem with this."