Bahrain's top Shiite cleric Sheikh Isa Qassim left Manama for treatment in London on Monday after his health deteriorated, his aides said.
The ayatollah, in his late 70s, was stripped of his Bahraini nationality in 2016 on charges of "serving foreign interests".
He flew out of the country at 10:30 AM (0730 GMT) using a temporary passport -- valid for one year -- issued on the orders of Bahrain's king, the aides said.
While the family has not made public the details of his condition, rights groups say the cleric has high blood pressure, diabetes and heart disease.
Bahrain's foreign minister tweeted Friday that the country's monarch had ordered the "facilitation of travel" for Qassim to seek treatment abroad.
The cleric's health has for months been a point of contention between authorities and his family, who had refused offers to transfer him to hospital over fears he could be detained and deported.
He did however undergo surgery in December before returning to de facto house arrest, activists said.
In January, a Bahraini court upheld the decision to revoke his nationality.
The Gulf nation's king has offered to cover the cost of "any treatment required", Bahrain's foreign minister said Friday.
Those close to Qassim, who has played a leading role in protests against Bahrain's Sunni monarchy, said the issue of payment had not yet been decided, and it was up to the sheikh to accept or reject the king's offer.
The monarch's olive branch to the influential cleric came less than three weeks after a Bahraini court made a rare acquittal of another Shiite opposition figure.
Sheikh Ali Salman, head of Bahrain's largest -- and now banned -- Shiite opposition group Al-Wefaq was found not guilty on charges of spying for regional rival Qatar.
Located between regional arch-rivals Iran and Saudi Arabia, Sunni-ruled Bahrain is home to a Shiite majority that has long complained of political marginalisation.
Security forces crushed demonstrations demanding a constitutional monarchy and an elected prime minister in 2011.
The country has been rocked by unrest ever since.
Manama has long accused Tehran of backing the protests and seeking to overthrow the government. Tehran denies any involvement.
Authorities have jailed dozens of high-profile activists, disbanded both religious and secular opposition groups and stripped hundreds of people of their citizenship.
The strategic Gulf kingdom is a key regional ally of the United States and serves as home for its Fifth Fleet.