Wickremesinghe's remarks came as he presented a report of the all party steering committee on formulating the Constitution in Parliament.
The new Constitution was a key promise of Sri Lankan President Maithripala Sirisena during the 2015 elections and is primarily meant to address the grievances of minorities.
The report will be debated both within and outside Parliament before the final bill is passed through a two-third majority followed by referendum.
The interim report has replaced the word 'unitary' with 'unified' but has refrained from proposing a federal status for country which Tamil political groups have been demanding.
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The TNA has been pressing for a federal solution to address political aspirations of the Tamil community, claiming discrimination by the majority Sinhalese.
The federal solution is being viewed as the moderate stance as opposed to the LTTEs separatist campaign of over three decades to create a separate Tamil state in north and east.
"This is only a draft, not a legally binding document. The final draft (constitution) would be done only after subjecting this report for public debate," Wickremesinghe said.
The TNA's R Sampanthan, who is also the main opposition leader, said this opportunity should not be missed and all parties must come together to bring in a new Constitution.
Sri Lanka wants to change the 1978 Constitution.
The proposal to end executive Presidency has not been agreed upon by major parties and Prime minister hoped that they will soon arrive at a consensus.
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