COLOMBO (Reuters) - Sri Lanka's president and opposition leader agreed in principle on Monday to endorse a constitutional change that would weaken the presidency and create an executive prime minister's post.

If implemented, the amendments would mean the sweeping and largely unchecked powers enjoyed by President Mahinda Rajapaksa would be vested in a premier accountable to the Indian Ocean nation's 225-member parliament.

The announcement marks a reverse of sorts for Rajapaksa, who has publicly expressed interest in changing the charter to allow himself a third term in office. However, he has also said he would consider running for an executive premiership.

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"The president is of the view that an executive prime minister is the ideal situation in terms of answerability," to parliament, Media Minister Keheliya Rambukwella told Reuters.

The president also agreed with Ranil Wickremesinghe, a former prime minister from 2001-2004 and the leader of the main opposition United National Party (UNP), to establish a joint committee to discuss on constitutional reforms.

"In principle, both the leaders agreed on the executive premiership and to discuss more on the constitutional amendments," UNP secretary-general Tissa Attanayake told Reuters.

Sri Lanka's last similar agreement on constitutional change, struck in 2000, fell apart at the last minute when the opposition under Wickremensinghe pulled its support.

The president's office said both parties agreed on Monday to change the 17th amendment, which was never put into practice but was designed to take away the president's power to unilaterally appoint judges and key public officials.

The president's ruling United Peoples Freedom Alliance won a parliamentary majority in April six seats shy of the two-thirds he needs to implement constitutional reform.

That commanding majority, coupled with Rajapaksa's appointment of three brothers to key positions, has sparked fears he would use the chance to change the charter to consolidate power and entrench his family's political dynasty.

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Rajapaksa, who served as prime minister in 2004-2005, has pledged to get rid of the executive presidency, widely criticised by Sri Lankans for being invested with too much power and too little legislative oversight.

From independence in 1948 until 1978, Sri Lanka operated under a Westminster-style of government, when then-Prime Minister J.R. Jayawardene used a five-sixths majority to adopt a new charter that made him the nation's first executive president.

(Editing by Andrew Marshall)