The British founder of the organisation that trained the Syrian rescue group known as the White Helmets has died in Istanbul.

A spokesman for the White Helmets confirmed on Monday afternoon the death of James Le Mesurier and said further details were yet to be established.

Le Mesurier, 48, was found dead near his apartment in central Istanbul’s Beyoğlu neighbourhood at around 4.30am. Turkish media reports said he was found with fractures to his head and legs and appeared to have fallen from a balcony.

The office of Istanbul’s governor said it had commenced a “comprehensive” investigation. The Reuters news agency said it had been told his death was being treated as suspected suicide.

Friends said Le Mesurier had been operating for years under significant pressure from orchestrated attacks on his reputation and the stress of running a high-profile NGO.

The former British army officer founded Mayday Rescue, a not-for-profit group that organised and trained the White Helmets, officially known as Syrian Civil Defence.

The group, made up of more than 3,000 volunteers, operates inside opposition-held areas in Syria and has been credited with saving the lives of thousands of people affected by Russian and Syrian airstrikes and bombings.

White Helmet workers rush to the scenes of bombings to try to rescue the wounded from the rubble. They have also helped to document alleged war crimes, including the use of chemical weapons.

Le Mesurier told the BBC in 2014 that the White Helmets sprang out of volunteer groups established in neighbourhoods across Syria that were under regime bombardment. He said he admired their work and sought to assist with training, fundraising and advocacy.

“These are ordinary individuals,” he said. “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.”

The group continues to operate in Idlib, the last opposition-held territory in Syria. Three of its volunteers were injured in Russian shelling that killed three people last week, it said.

The Guardian has previously documented how the organisation has been the target of a disinformation campaign, conducted with the support of the Russian government, that positions it as an al-Qaida-linked terrorist organisation.

Le Mesurier had been subject to similar campaigns and was targeted by Russia’s foreign ministry as recently as last week.

#Zakharova: The White Helmets’ co-founder, James Le Mesurier, is a former agent of Britain’s MI6, who has been spotted all around the world, including in the #Balkans and the #MiddleEast. His connections to terrorist groups were reported back during his mission in #Kosovo. pic.twitter.com/Fa7JTuP39Z — MFA Russia 🇷🇺 (@mfa_russia) November 8, 2019

The NGO’s funders currently include the British and German governments. The Trump administration froze US funding, which made up about one-third of the total, without public explanation in early 2018, but resumed giving financial aid last month amid criticism of its decision to withdraw US troops from north-eastern Syria.

Le Mesurier, whose father was a commandant in the Royal Marines, attended the Royal Military Academy Sandhurst and served in the armed forces, including in the Balkans, until the late 1990s. He worked in various security consulting jobs before creating Mayday Rescue in 2014.

Mayday Rescue said in a statement that Le Mesurier had dedicated his life to helping civilians, especially in Syria. “We ask that James be remembered as what he was: a great leader, a visionary, and a dear colleague and friend,” it said.

Syria Civil Defence expressed “our deepest sorrow and solidarity” with Le Mesurier’s family. “We also must commend his humanitarian efforts which Syrians will always remember,” it said on Twitter.

The White Helmets have been nominated for the Nobel peace prize several times and a documentary about the group won an Oscar in 2017.

Le Mesurier received an OBE in 2016 for his services to Syrian civilians. Around 100 members of the White Helmets have been resettled in the UK, according to the Home Office, after they were evacuated from Syria in the face of advances by the regime.

The UK Foreign Office said: “We are deeply saddened by the news of the death of James Le Mesurier. Our condolences go out to his family and friends at this difficult time.”

• In the UK and Ireland, Samaritans can be contacted on 116 123 or email jo@samaritans.org or jo@samaritans.ie. In the US, the National Suicide Prevention Lifeline is 1-800-273-8255. In Australia, the crisis support service Lifeline is 13 11 14. Other international helplines can be found at www.befrienders.org.