The Tamil National Alliance (TNA) deliberated in Jaffna yesterday and arrived at this decision, the sources said.
The decision has come in the context of current speculation that the TNA may support Prime Minister Ranil Wickremesinghe to continue his government.
After its electoral debacle in local council elections, Wickremesinghe's United National Party (UNP) has run into problems with the premier facing pressure to give up the party leadership.
The unity government is even facing the prospect of collapse following the Saturday's election results in which former president Mahinda Rajapaksa's new party Sri Lanka People's Party (SLPP) recorded a resounding victory.
SLFP suffered its worst drubbing with just 13 per cent of the vote.
The UNP is looking to form a government of its own despite having only 106 seats, 7 short of the working majority.
The TNA had supported the unity government's opposition coalition against Rajapaksa in 2015 and Sirisena was elected on the party's support from the Tamil-dominated north.
Despite the moderate Tamil support for Sirisena's reconciliation, Tamil nationalists remain disgruntled with the slow pace in the delivery of a new Constitution which will address the political aspirations of the community.
Rajapaksa's large support from the Sinhala Buddhist majority appears to make the Constitutional reforms effort a non-starter.
Last year, the TNA had said it will not be a part of the ongoing Constitution-making process and will quit if the government abandoned the idea of finding a political solution to the Tamil issue and more devolution was not considered.
The new Constitution will replace the current executive president-headed Constitution adopted in 1978.
The government expects the new Constitution to address the demand of Tamil minorities for political recognition.
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