CAIRO (REUTERS) - More than 98 per cent of Egyptians backed a new constitution in a referendum, authorities said on Saturday, though the turnout was lower than some officials had predicted, with just over a third of the electorate taking part.

The vote advances a transition plan the military-backed government unveiled after deposing Islamist president Mohamed Morsi last July following mass unrest over his rule.

Voters supported the constitution by 98.1 per cent, said the head of the supreme election committee, Nabil Salib. A total of 38.6 per cent of eligible voters took part, he told reporters.

An Interior Ministry official had said on Thursday the turnout appeared to be more than 55 percent in what was the first vote since Morsi's overthrow.