"We aim to expand our political alliance further by getting more groups into our fold," United National Party (UNP) general secretary Tissa Attanayake told reporters.
He said that varied parties would be invited to come together on broader national issues.
"We have already identified areas of common interest such as abolition of the executive presidential system and reviving the seventeenth amendment," Attanayake said.
In 2012, UNP formed an alliance named 'Opposition's Protest' with several smaller political groups and now the general secretary has said its expansion will take place in August, ahead of the provincial elections in September.
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The opposition party, which played a key role in adoption of the present constitution in 1978, recently introduced a constitutional reform package aiming at disbanding the current system of government with president as the head.
Also, UNP has called on Lankan President Mahinda Rajapaksa to revive the seventeenth amendment of the constitution, which focuses on depoliticisation of key institutions such as the judiciary, election commission, police and the public services.