BEIRUT, Lebanon — A British man who was a critical player in creating and sustaining the Syrian civil defense group known as the White Helmets was found dead in Istanbul on Monday, a group official said.

The Briton, James Le Mesurier, was the founder and director of Mayday Rescue, a nonprofit group that has supported, funded and trained the White Helmets, a volunteer group that has gained international renown for rescuing wounded civilians during Syria’s civil war.

Mr. Le Mesurier’s body was found early Monday in the Beyoglu neighborhood of Istanbul, outside a building where he kept an apartment and office, said Raed Saleh, the lead organizer of the White Helmets.

“James was a real friend, and it’s a big loss,” Mr. Saleh said. He said that Mr. Le Mesurier’s wife had told the organization about his death.