The Democratic Party has given the formal go-ahead for its leader, Marlene Farrugia, to join the Opposition parliamentary group.
"This will ensure that there will be a more coordinated and effective parliamentary opposition," it said.
The PN and the PD had contested the general election in a coalition, with the PD candidates listed as PN candidates on ballot papers.
PD said its participation in the opposition parliamentary group would make it more effectively, constructive and proactive in the House debates and the fulfilment of all its parliamentary obligations, in particular, the scrutiny of the government.