"There is now an urgent need to restore the dignity and sovereign rights to the people which were forcibly taken away by the present regime and to adopt a new constitution," said United National Party (UNP) leader Ranil Wickremesinghe.
He alleged that the current Mahinda Rajapaksa-led administration has completely desecrated the office of the President by destroying all checks and balances and creating an authoritarian regime by "blatant abuse of power".
Under the Westminster system the Cabinet of Ministers will be restricted to 25 including the Prime Minister. The party also mulls retaining the existing provincial councils system as means of devolving power.
The proposed constitution of the UNP also guarantees the freedom of expression and will be strengthened to include the right to information.
UNP's bid to abolish the presidential system follows threats by a powerful Buddhist monk-led civil movement to field a common opposition candidate in the next presidential election in 2016.
The movement accused the two main political parties of Lanka, the UNP and the ruling UPFA coalition, to be hesitant to abolish the executive presidency.