Some 12 percent of respondents were undecided, underscoring significant potential for volatility, Ipsos said in a statement published on its website, describing the referendum as "too close to call".

The poll was the fifth released on Friday, and the fourth giving a narrow lead to the 'Yes' camp in a plebiscite that may decide the country's future in the euro zone.

(Reporting By Michele Kambas; Editing by Matt Robinson)