The euro was last at $1.4410, up 0.3 percent on the day, but off the session peak of $1.4447, according to Reuters data. The session low posted at $1.4325.

"We should get a euro relief rally if (the vote) passes, as the sell-off just now reflects just how nervous people are that it might not pass," said Kathy Lien, director of research at GFT Forex. "Expect a lot of volatility and position adjusting in the meantime."

Copyright Reuters, 2011