The body of James Le Mesurier was discovered near his apartment in Istanbul's Beyoglu neighborhood around 4:30 a.m., the state-run Anadolu news agency reported.
Worshipers found Le Mesurier as they were on their way to a mosque. Officials told Reuters the death was being treated as a possible suicide because he may have fallen from his balcony. Local media reports said Le Mesurier had fractures to his legs and head.
The 48-year-old was the CEO of May Day Rescue, which founded and trained the White Helmets, also known as the Syria Civil Defense. He moved to Turkey with his wife around four years ago.
The group confirmed Le Mesuire's death on its Facebook page.
"The Syrian Civil Defense family extends its deepest condolences to the James family, and we express our deepest sorrow and solidarity with his family," it posted. "As we also must commend his humanitarian efforts which Syrians will always remember."
The White Helmets have more than 3,000 volunteers working in opposition-held areas in Syria. They often rush to areas where bombings have occurred and give aid to the injured. They have been credited with saving thousands of civilians caught up in attacks and documenting alleged war crimes.
"These are ordinary individuals, Le Mesurier told the BBC in 2014, according to the Guardian. "Former bakers, former builders, former students who had choices for what they were going to do with their lives within the revolution. These individuals chose to stay, with very little equipment and at the beginning with no training whatsoever, to respond to bomb attacks, to respond to shellings and try to save their fellow Syrian civilians.”
Karen Pierce, Britain's ambassador to the U.N., called Le Mesurier a "true hero."
"He really deserves our respect," she said. "The world, and Syria in particular, is poorer for his loss."
Le Mesurier said the White Helmets were bourne out of volunteer groups operating in areas under assault from Syrian President Bashar al-Assad. The group has been nominated for the Nobel Peace Prize several times.
A 40-minute documentary about the White Helmets won an Academy Award in 2017 in the short-subject category.
Last week, Russian Foreign Ministry spokeswoman Maria Zakharova had accused Le Mesurier of being a former British agent who has "been spotted all around the world, including in the Balkans and the Middle East."
"Given the role of the West in undermining stability in these regions, it is not difficult to assume what the British intelligence officer did there," she said.
The DHA news agency in Turkey said authorities were investigating Le Mesurier's death as a suicide and whether he had been taking antidepressants.
Anadolu reported that his wife told police they had been up until 4 a.m. and then took sleeping pills. She was woken by a doorbell less than twp hours later and saw her husband's body from a window in their third-floor apartment.
Le Mesurier is expected to undergo an autopsy, DHA reported.