Another member of Tonga's Friendly Islands Democratic Party is to take up the vacant Health portfolio following the sudden resignation of the party leader, Akilisi Pohiva.
Uliti Uata expects to be sworn in this week.
He will join Isileli Pulu who was named in the cabinet by the prime minister, Lord Tu'ivakano, last month.
Mr Pohiva pulled out in protest at the prime minister adding two unelected people to his cabinet.
Our correspondent, Mateni Tapueluelu, says the party members have backed this move, saying it's best if their leader is not bound by the memorandum of understanding applying to cabinet ministers.
"Where they cannot challenge or criticise the prime minister. In his position as leader of the opposition it is best that he pulls out and gathers together the rest of the party members, which is the majority outside, as opposition. So for effective opposition it is best that he stays outside of the cabinet."