An alliance of Egyptian opposition parties said on Tuesday they would boycott forthcoming parliamentary elections in protest at an election law that they say favors the Muslim Brotherhood, increasing the chance that Islamists will sweep the vote.

The boycott by liberal and leftist parties opposed to President Mohammed Morsi raises the prospect of a parliamentary election fought out mostly between his Muslim Brotherhood and more hardline Islamist groups such as the Salafi Nour Party.

The vote is to be held in four stages, starting in late April.

Seeking to undermine the legitimacy of the elections to the lower house of parliament, the National Salvation Front (NSF) said there could be no elections without a law guaranteeing free and fair polls.

Critics of the election law passed by the Islamist-dominated upper house of parliament say it has been skewed to favor Islamists.

"There can be no elections without a law that guarantees the fairness of the election process and a government that can implement such a law and be trusted by the people," said Sameh Ashour, a spokesman for the NSF, adding that there must also be "real independence of the judiciary."

The NSF includes the Popular Current, a leftist party led by Hamdeen Sabahy. Party spokeswoman Heba Yassin said the boycott was "to protest against the election law that we did not participate in drafting, and about which our opinion was not taken".