A secret British team of ex-soldiers and engineers has saved a Christian town in Lebanon from the advances of the Islamic State [IS] by building a network of watchtowers along the Syrian border.

In July, the squad quickly constructed 12 lookout points for the Lebanese army to stem IS's march across the Middle East towards the country, protecting the Christian town of Ras Baalbek in the process.

Tom Fletcher, the British ambassador to Lebanon, told The Telegraph that the watchtowers built by the British squad had thwarted a "massacre" in the town.

"They [IS] want these big symbolic victories — you bust through a border, you carry out a massacre and you get the attention," he said.

"In a country that has such existing fragilities, that would have had dramatic consequences."

The town is home to a population of around 5,000 people who are mainly Christian. Lebanon itself holds a population of two million Christians who live alongside a number of Druze, Sunni and Shia Muslims.

The watchtowers were each constructed out of six shipping containers, and more towers are now being planned for the border. The tower known as Tango 10, which overlooks Ras Baalbek, cost the British taxpayer £150,000, according to a former soldier with involvement in the project.

The British team spoke of the basic nature of the security on the Lebanese-Syria border when they arrived to construct the watchtowers.

"You can't imagine what it was like when we got out here first," says one former officer. "There were a few guys behind some tyres filled with rocks with a 50 cal [machine gun]."

"You have to start security somewhere," explains a former British officer. "We are not building the Maginot line. People are scared. They've all seen the decapitations on YouTube. They know what Isil [IS] want to do."

Faris Mansour, a 76-year-old Christian from the town, told The Telegraph of the town's inhabitants' gratitude for the British help.

"During the time of the crisis, the watchtowers helped to protect us," he said.

"We were concerned about what was happening in Iraq and Syria. We were watching the news all the time, and we thought the violence would happen here."