Bahrains top Shiite cleric Sheikh Isa Qassim left Manama for treatment in London on Monday following a deterioration in his health, his aides said.
Qassim was allowed to fly out of the country using a temporary passport -- valid for one year -- issued on the orders of Bahrain's king, the aides said.
Bahrain's foreign minister tweeted Friday that the country's monarch had ordered the "facilitation of travel" for the cleric to seek treatment abroad.
The Bahraini king also offered to cover the cost of "any treatment required", the minister added.
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.
Qassim's health has for months been a point of contention between authorities and the Qassim family, who long refused the government's offer to transfer him to a local hospital over fears he could be detained and deported should he leave his residence.
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, and the country has been rocked by ongoing unrest since.
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.