Mr Haines, a 44-year-old father of two, had been held by self-styled Islamic State fighters.

He was the third Western hostage - and first Briton - paraded on camera by the extremist organisation.

Mr Haines was working for a French aid agency when he was kidnapped in Syria in March 2013.

He joined ACTED to help co-ordinate the delivery of clean water, food and tents, in order to ease the growing humanitarian crisis in refugee camps near Atmeh, a town in northern Idlib province, near the Syrian border with Turkey.

Mr Haines and other aid workers were staying in Atmeh - and on 12 March he and an Italian colleague, Federico Motka, were kidnapped. Mr Motka was later released.

Born in Holderness, East Yorkshire, Mr Haines was raised in Scotland and attended Perth Academy. His parents live in Ayr.

After leaving school, he worked for Royal Mail, before joining the RAF as an aircraft engineer.

Mr Haines served in the UN mission in the Balkans, where, according to a family statement, he "helped whoever needed help, regardless of race, creed or religion".

After leaving the RAF, he worked for Scotrail, before beginning his career in humanitarian work.

His postings for international aid agencies took him to some of the most dangerous places in the world.

Mr Haines's online business profile also says that he worked for a US company supplying consumer goods to the military around the world - although the company has declined to comment.

Croatian reconstruction

Between 1999 and 2004 he worked for Arbeiter Samariter Bund (ASB), a Germany charity carrying out reconstruction work in post-war Croatia.

Mr Haines was the head of a regional office for the charity charged with managing a large European Commission-funded plan to help hundreds of displaced people return to the country and recover and rebuild their homes. He was involved in other projects in Croatia such as rebuilding a kindergarten in Benkovac.

A man's life should never be threatened on account of his humanitarian commitment”

Shortly after news emerged that Mr Haines was the British hostage in Syria, ASB published a statement on its website saying: "In these difficult times our thoughts are with our colleague and friend David Haines who is in captivity.

"We pray for his safe return and call upon the International Community to help save the lives of innocent aid workers."

In April 2011, Mr Haines joined another charity, Handicap International, and became the head of its mission in war-torn Libya. The organisation, which carries out demining work around the world, sent a team to the country to help educate people about the dangers posed by the weapons and other explosives.

The following year, he moved on to South Sudan, where he worked with Nonviolent Peaceforce, an organisation seeking to facilitate peace initiatives in dangerous environments.