James Le Mesurier, White Helmets co-founder, discovered dead in Turkey
The body of James Le Mesurier, who received an OBE in 2016, was discovered on Monday near his home in Istanbul, White Helmet sources told the BBC.
Mr Le Mesurier set up the Mayday Rescue emergency response group, which helped train White Helmets volunteers.
The cause of death is not known. Turkey has launched an investigation.
In a tweet, the White Helmets said the group had "learned with shock and sadness the news" of the death of Mr Le Mesurier.
Hamish de Bretton-Gordon, a director of the Doctors Under Fire campaign group and a friend of Mr Le Mesurier, said: "It's absolutely tragic. He was one of the few people who have made a humanitarian footprint in Syria."
Mr de Bretton-Gordon said the White Helmets had a very "strong structure" and their work would continue. But he said Mr Le Mesurier's death had left a "hole to fill".
What do we know about Le Mesurier's death?
Mr Le Mesurier's body was found at about 04:30 local time (01:30 GMT) on the street near his home and office in Istanbul's Beyoglu district on the European side of the city.
He was found with fractures to his head and legs, Turkish media say, and is believed to have fallen from his balcony.
A "comprehensive" investigation has been launched, the Istanbul governor's office says.
Who was Le Mesurier?
Mr Le Mesurier, who was believed to be in his 40s at the time of his death and had also worked for the United Nations, was considered a co-founder of the White Helmets.
The organisation, which is also known as the Syria Civil Defence, helps rescue civilians caught up in attacks in areas of Syria controlled by the opposition to President Bashar al-Assad.
In 2016, the organisation received the Right Livelihood Award in recognition for "outstanding bravery, compassion and humanitarian engagement in rescuing civilians". Later the same year the group was nominated for the Nobel Peace Prize.
But the Syrian government and its allies Russia and Iran have accused the White Helmets of openly aiding terrorist organisations and the Russian foreign ministry last week accused Mr Le Mesurier of being a former agent of the UK's Secret Intelligence Service, better known as MI6.
The Russian allegation was strongly denied on Monday by Karen Pierce, the UK ambassador to the UN, who said: "The Russian charges against him, that came out of the foreign ministry that he was a spy, are categorically untrue."
Mr Le Mesurier received an OBE (Officer of the Order of the British Empire) from the Queen in 2016 for "services to the Syria Civil Defence group and the protection of civilians in Syria"
Who are the White Helmets?
- Started in 2014 as a volunteer workforce, run by donations
- Known for their distinctive white headwear
- Include former bakers, tailors, carpenters and other professions
- The group says it has saved more than 100,000 people
- It says 252 members have been killed and more than 500 injured
- Nominated for the Nobel Peace Prize in 2016
- Say they are neutral, have no political affiliation and save people from all sides of conflict
- Also do repair works, reconnect electrical cables and secure the buildings
- Seen as a terrorist group by Syria's government