Statistics show that divorce rates have risen since the advent of Facebook and it has sharply increased marital infidelity, Sheikh Abdel Hamid al-Atras said.
"It's an instrument that destroys the family because it encourages spouses to have relations with other people which break Islamic sharia law," said al-Atrash, quoted by pan-Arab daily Al-Sharq al-Awsat.
He is the former head of the fatwa commission at Cairo's Al-Azhar University.
"While one or other of the spouses is at work, the other is chatting online with someone else, wasting their time and flouting the Sharia. This endangers the Muslim family," said al-Atrash.
The edict followed the publication earlier this week of a study claiming one out of five of divorces in Egypt had been caused by liaisons begun on Facebook or other social networking sites.
Like satellite TV, social networking sites are a "double-edged sword", al-Atrash said.
"While they permit the spread of Islam, they allow people forbidden love and relations.
"That is why whoever uses such websites must be considered a sinner," he said.
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