Iraq's most influential Shia leader, Grand Ayatollah Ali Sistani, has arrived in London to be treated for a heart condition.
The ayatollah represents the conservative mainstream of Iraqi Shias
A spokesman for the ayatollah said he was very tired after flying via Beirut and would be resting.
The ayatollah lives in the holy city of Najaf and has been receiving treatment from cardiologists.
But aides have said they fear the violence in Najaf could hamper his access to proper medical care.
"The ayatollah suffered a health crisis related to his heart a few days ago," his spokesman in Beirut, Sheikh Hamed Khafaf, told the Associated Press news agency.
"A team of specialist doctors are treating him and providing care," he said of the ayatollah, who is the prime marja, or spiritual reference, for Shias.
He added that although the ayatollah needed special treatment, he was not in a serious condition.
Fighting broke out once more in the ayatollah's home town of Najaf on Thursday.
Clashes continued on Friday as US helicopters attacked targets in the holy city.
The reclusive Ayatollah Sistani, who is in his 70s, is one of only five living grand ayatollahs.
The UK Foreign Office confirmed that Ayatollah Sistani was visiting the UK on a private medical trip for which the UK would make no special security arrangements.
A Foreign Office spokesman said there was no political aspect to the trip, but that the FO had helped out with the logistical elements like visas.