Egypt state TV said the court issued its ruling on Monday.
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The Brotherhood was outlawed for most of its 85 years in existence. But after the 2011 ouster of autocrat Hosni Mubarak, it was allowed to work openly, formed a political party and rose to power in a string of post-Mubarak elections. In March, it registered as a recognized non-governmental organization.
The army-backed government is waging the toughest crackdown in decades on the Islamist group, which says it has a million members. Security forces killed hundreds of its supporters and rounded up thousands more since Morsi was deposed by the army on July 3 after mass protests against his rule.
Army chief Abdel Fattah al-Sisi's toppling of Morsi triggered a political crisis in the Arab world's most populous state, with the Brotherhood insisting that a military coup robbed them of power.
The court decision is likely to drive more Brotherhood members underground and it may encourage young Islamists to take up arms against the state.
The ruling, which can be appealed, opens door for authorities to track down the group's elaborate network of social services, dealing a deadly blow to its pillars of grass-root support.
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