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Bahrain's top Shia cleric Sheikh Isa Qassim left Manama for treatment in London on Monday following a deterioration in his health, his aides have said.

The ayatollah, who was stripped of his Bahraini nationality in 2016 on charges of "serving foreign interests", was allowed to fly out of the country using a temporary passport issued on the orders of Bahrain's king, the aides said.

The cleric's ailing health has for months been a point of contention between authorities and the Qassim family, who had refused the government's offer to transfer him to hospital over fears he may be detained and deported should he leave his residence. 

Sheikh Qassim, in his late 70s, spearheaded Bahrain's opposition movement demanding an elected government in the Shia-majority kingdom, ruled for more than two centuries by the Sunni al-Khalifa dynasty.

He has been under de facto house arrest since 2016.

The kingdom's Shia population has long complained of marginalisation and the country has been rocked by sporadic unrest since 2011.

The protesters have called for a constitutional monarchy and an elected prime minister. The protests - led by Bahrain's Shia population - were brutally suppressed by authorities and a Saudi-ked intervention force.