The package of 18 constitutional changes that was passed by parliament last weekend is yet to be submitted to President Recep Tayyip Erdogan for approval, Yildirim told an Ankara meeting of Justice and Development (AK) Party provincial heads.
“If he approves the amendment, the referendum could be held in the first half of April,” he said.
The referendum must be held on the first Sunday that falls 60 days after a notification is published in the Official Gazette. Such a notification will come once the bill is approved by Erdogan -- making April 2 the likeliest date.
Last week, Deputy Prime Minister Numan Kurtulmus suggested April 2 or 9 as a possible referendum date.
The changes effectively shift Turkey from a parliamentary system to a presidential one by making the president the head of the executive and abolishing the prime minister’s role.
The reforms also allow the president to be a party leader, call elections, declare a state of emergency and issue legally-binding decrees.
If passed, they will come into effect when presidential and parliamentary elections are held in November 2019.
Yildirim said the proposed new system would strengthen Turkey politically and economically and increase its international influence. He rejected opposition claims that it amounted to a change of regime.
“This constitutional change will bring permanent stability, peace in the country,” he told AK Party leaders.
He said Erdogan’s strong leadership had boosted its regional profile. “While Turkey was excluded from Syria negotiations one year ago, now the country sits in a leading position,” he said.
With Russia, Turkey helped broker a cease-fire in Syria and peace negotiations between the warring sides on Monday and Tuesday.
In a wide-ranging speech, Yildirim added that the military operation in northern Syria backed by Turkish troops would give Syrian refugees in Turkey the security to return home.
“The main goal of Operation Euphrates Shield is to clear terrorist groups from the region and ensure a secure place for our Syrian brothers,” he said.
The premier added: “We host them in their difficult days but after providing security and peace in Syria they will return to their homeland.”
Turning to east and southeast Turkey, parts of which have been affected by PKK terrorism, Yildirim called for extra investment and promised government support through infrastructure projects.