"Prime Minister (Hazem El) Beblawi has declared the Muslim Brotherhood as a terrorist organisation," state-run news agency MENA quoted the premier's spokesperson Sherif Shawki as saying.
The condemnation of the Muslim Brotherhood - considered the largest and best-organised political force in Egypt, came weeks ahead of a referendum on a new constitution that is described as a major step toward returning to democracy since the military removed President Mohammad Mursi in July.
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An Egyptian court has already outlawed the activities of the Brotherhood. The military-backed interim authorities have often accused the Islamist movement of funding militants in the country.
Since the ouster of Muslim Brotherhood leader Mursi - the country's first freely elected president - his supporters have been staging rallies demanding his reinstatement. Over 2,000 Brotherhood members have been arrested, including the movement's supreme guide Mohamed Badie.
Premier Beblawi's remark came just hours after a huge blast at a security building in northern Mansoura city left at least 14 people dead and over 100 injured. No group has so far claimed responsibility for the blast.
MENA quoted Beblawi as saying that those responsible for blast "will not escape punishment".
"The majority of the casualties are from the police. The explosion was caused by a car bomb," Omar al-Shawatsi, the governor of Daqahleya, of which Mansoura is the capital, told the state media. The provincial security chief was among those wounded, reports said.
The Muslim Brotherhood also condemned the bombing. "Muslim Brotherhood considers this act as a direct attack on the unity of the Egyptian people," it said in a statement.