* Former army head expected to win May 26-27 presidential vote
* Cleric says voting for Sisi would be "a great sin"
* Some of Qaradawi's sermons may have contributed to Gulf rift
DOHA, May 25 (Reuters) - The influential Qatar-based Muslim cleric Sheikh Youssef al-Qaradawi on Sunday called on Egyptians to boycott presidential elections and shun front-runner Abdel Fatah al-Sisi, saying the former army chief had "disobeyed God".
The Egyptian-born cleric, who has close links to the Muslim Brotherhood and whose religious shows on Al Jazeera television were watched by millions, has been critical of Egypt's military-backed government, accusing Sisi of betrayal for ousting Islamist President Mohamed Mursi last year.
Qaradawi's outspoken support for the Brotherhood has recently contributed to an unprecedented diplomatic rift between Qatar and its Gulf Arab allies who consider the Islamist group a security threat and supported Mursi's overthrow.
"People of Egypt in the capital and the provinces, cities and villages, sit in your houses and do not burden yourselves with a great sin...," Qaradawi said in an emailed statement.
"It is not permissible for you to vote for he who has disobeyed God," he added.
Sisi is expected to easily win the May 26-27 presidential election. His only challenger is leftist politician Hamdeen Sabahi, who came third in the 2012 vote won by Mursi.
Since Mursi was ousted, the new military-backed government has cracked down hard on the Brotherhood, once the country's best organised force, accusing members of fomenting violence and unrest. Thousands were killed or rounded up by security forces.
The Brotherhood accuses Sisi of staging a coup and masterminding the removal of Mursi, who was Egypt's first freely elected president.
"The duty of the nation is to resist the oppressors, restrain their hands and silence their tongues," said Qaradawi, who has previously said he only supports peaceful resistance in Egypt.
"I refuse to participate in the election... do not go to participate in the injustice," he said.
Qaradawi said that Sisi's victory in the poll would please the "Zionists" and "enemies of the nation" -- an apparent reference to Israel.
Gulf Arab states have been unhappy with Doha for sheltering Qaradawi, a critic of Saudi and UAE authorities, and for giving him air time on its pan-Arab satellite and state television channels.
Saudi Arabia, the UAE and Bahrain were especially angry over Qatar's support for the Muslim Brotherhood, whose ideology challenges the principle of conservative dynastic rule that dominates the Gulf.
Eventually the three states recalled their ambassadors from Qatar in March, accusing Doha of failing to abide by an accord not to interfere in each others' affairs. Qatar denies the charge.
Since then, Qaradawi has refrained from delivering Friday sermons. But this has not stopped him from criticizing Egypt's rulers during conferences or by statements often sent by email. (Reporting by Amena Bakr; Editing by Sami Aboudi and Raissa Kasolowsky)