Civilian drone
As civilian drones can reach altitudes of 5,000ft, fears grow they could be used to carry out a terrorist attackGetty

Passenger planes are running a constant risk of being downed by drones being flown too close to them, a government counterterrorism expert has warned. The unmanned aircraft could also be used to conduct terrorist attacks and for propaganda purposes at high-profile events.

Over the past year, there have been a string of fatal near-misses with commercial carriers with drones coming within a matter of feet of the passenger-carrying jets. In March 2015, a plane came within just 20ft of a drone as it prepared to land at Heathrow Airport. Drones – which can be purchased for just £30 – have been seen near aircraft at altitudes of up to 5,000 feet.

"This is happening on a weekly basis," Home Office counter-terrorism expert Colin Smith said. "We are not just talking about hundreds of feet, we are talking thousands. People are flying drones in the path of aircraft, having a look, seeing if they can get alongside the cockpit to see what is going on."

Aside from accidental contact, Smith raised the possibility of terrorists deliberately flying drones into a jet's engines while it is in mid-air.

Smith also warned that drones could be used to spread terror-related propaganda, referring to the example of a low-flying drone bearing the insignia of a so-called "Greater Albania" during a Euro 2016 qualifying match between Serbia and Albania. Play was halted in the 2014 game between the two sides as the incident sparked violence, with Serbian supporters invading the pitch and attacking Albanian players.

"As an embarrassment factor, change that to an Isis flag and fly that down Whitehall during Remembrance Sunday," Smith told the Sunday Times. "Who's going to be able to stop that flag being flown? It is a real and current problem," he added.

The government has faced calls by pilots to tighten rules on the civilian use of drones as safety concerns continue to mount. Four near misses with drones had been recorded in 2014 and the figure more than trebled to 15 confirmed cases by September 2015, according to the Mirror.

"Drone users have to understand that when taking to the skies they are entering one of the busiest areas of airspace in the world — a complex system that brings together passenger aeroplanes, military jets, helicopters, gliders, light aircraft and now drones," the Civil Aviation Authority (CAA) said.

"The rules for flying drones are designed to keep all air users safe. Anyone flouting these rules can be charged and prosecuted and any drone user found guilty of endangering an aircraft could face imprisonment."