President Maithripala Sirisena presided over the swearing in ceremony of 43 ministers, who were included in the Cabinet for now with the provision to appoint a total of 48 members.
Parliament yesterday approved enhancing of the Cabinet members from the constitutionally provisioned 30 members to 48.
Sirisena's United People's Freedom Alliance (UPFA) and Prime Minister Wickremesinghe's United National Party (UNP) have struck a deal to forge the country's first national unity government.
Out of the UPFA's 11 ministers, there were a few loyalists of former President Mahinda Rajapaksa.
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"This is a historic day. The two main parties have resolved to work together for two years. We have to find solutions to many burning issues," Prime Minister Wickremesinghe said.
The Rajapaksa loyalists have publicly vowed to sit in the opposition in defiance of the national unity government arrangement.
Sirisena defeated Rajapaksa in the presidential election held in January and succeeded him as the party leader.
The move to expand the Cabinet was required as the 19th amendment to the Constitution adopted in April had restricted the Cabinet to 30 members.
Despite the constitutional restriction, the national unity government arrangement between Wickremesinghe's UNP and President Sirisena's UPFA meant that more portfolios were needed to be shared between the two parties.
The parties had agreed to form the national unity government following Wickremesinghe's victory in the August 17 election.