ISTANBUL (Reuters) - A narrow majority of Turks will vote “Yes” in Sunday’s referendum on changing the constitution to grant President Tayyip Erdogan sweeping new powers, two opinion polls showed on Thursday.

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The April 16 vote will decide on the biggest change in Turkey’s system of governance since the modern republic’s foundation almost a century ago, potentially replacing its parliamentary system with an executive presidency.

Polling company Konda said the number of “yes” voters stood at 51.5 percent, but said its survey had a margin of error of plus or minus 2.4 percent.

“When this forecast is considered within the survey’s margin of error, a final judgement might be misleading,” Konda said in a statement.

Its survey, carried out face-to-face with 3,462 people in 30 provinces on April 7-9, showed turnout for the vote would be around 90 percent. It said the level of undecided voters had fallen to 9 percent from more than 20 percent in January and there was no evidence to indicate their preference..

The survey by pollster Gezici put support for the constitutional change at 51.3 percent, with “no” votes at 48.7 percent after the distribution of undecided voters.

The poll was carried out face-to-face with some 1,400 people in 10 provinces on April 8-9. In its previous survey a week earlier it put the “yes” vote at 53.3 percent.

Gezici said many people did not want to express their views during the poll’s fieldwork. It put the level of undecided voters at 9.9 percent.

“One of the basic problems with the work during the referendum process was the evident increase in the level of people’s concern about expressing themselves,” Gezici said in its analysis of the results.

Two other surveys on Wednesday showed the “yes” vote on 51-52 percent. The mean average of nine polls collated by Reuters puts the “yes” vote on 50.9 percent

The referendum campaign has damaged Turkey’s ties with some European allies. Erdogan has described the banning on security grounds of some rallies by Turkish ministers in the Netherlands and Germany as “Nazi-like” tactics.

Voting for Turks living abroad finished on Sunday and Erdogan said this week that those overseas had turned out in greater numbers, a development that pollsters say could benefit him.