Egyptian cleric Youssef el-Qaradawi issued a fatwa, or religious edict, a day before Egyptians abroad are set to vote on the draft charter. Voting in Egypt is scheduled to start Jan. 14. Of Egypt's roughly 681,000 registered voters abroad, about 42,000 are based in Qatar.
His fatwa was issued amid growing tensions between Egypt and Qatar. Qatar was once a staunch supporter of Islamist President Mohammed Morsi and his Muslim Brotherhood group, aiding his government with billions of dollars in grants and loans during his tumultuous year in office.
The previous Egyptian constitution approved under Morsi was suspended after the military removed him from power following mass protests in June.
El-Qaradawi, who often speaks out against Egypt's new rulers, said Morsi's ouster was a military coup, not a revolution as his detractors have called it. He said in his fatwa that the coup was responsible for the killing and arrest of thousands of people.
El-Qaradawi, a frequent guest on the Qatar-based Al Jazeera network of channels, said that participating in the upcoming referendum constitutes a continuation of injustice and oppression.
"I see that participation in the constitutional referendum and contributing in any way that strengthens this coup authority, or gives it legitimacy, or prolongs its existence.
Over the weekend, Egyptian officials summoned Qatar's ambassador to Cairo to express concerns about the Gulf state's criticism of Egypt's decision to designate the Muslim Brotherhood a terrorist organization. The move was followed by a small rally outside Qatar's Embassy in Cairo on Tuesday by military supporters who raised photos of Egypt's defense minister and chanted against the Brotherhood and the tiny Gulf state.
El-Qaradwi was unusually absent from his traditional Friday sermon in Doha last week, prompting media reports he may have been asked by Qatar authorities to stop until relations with Egypt improve. Qatar's foreign policy has also been at odds with other Gulf countries like Saudi Arabia, Kuwait and the United Arab Emirates, which view the Brotherhood and its allied Islamist groups as a threat.