Police and firefighters are checking a British Airways plane on the tarmac of Paris' Charles de Gaulle airport after reports of a problem.
One passenger on the plane said it was surrounded by dozens of armed officers and firefighters.
James Anderson, a 20-year-old entrepreneur on the Sunday morning flight from Paris to London's Heathrow Airport, told The Associated Press (AP) that the pilot initially told passengers there were technical issues.
It is believed the plane was due to land in London Heathrow at 7.45 this morning.
Anderson said after about an hour, passengers were told the aircraft had to move to another part of the airport and that's when security officers surrounded the plane.
Anderson says "the pilot then said there had been a direct security threat involving our flight."
In a series of tweets, he wrote: "On British Airways flight BA0303, currently being held on tarmac at Paris due to security threat, surrounded by police and fire vehicles.
"Apparently an individual has made a direct threat to this aircraft. We will all be led off the aircraft & baggage searched in due course. Everybody's been individually searched by armed officers and given all clear. Now hold luggage being searched by dogs.
A British Airways spokesperson said: "The safety and security of our customers and crew is always our top priority. Additional security checks are being carried out as a precaution.
"We would never operate a flight unless it is safe to do so."
A spokesman for France's national gendarme service said police and firefighters rushed to the scene after receiving a "security alert."
The spokesman said the plane was evacuated, each passenger and each bag was checked and the plane was thoroughly examined but no threats were found.
Authorities determined it was a false alarm.
He would not elaborate on the nature of the original alert. The spokesman was not authorised to be publicly named according to gendarme policy.
In the UK, the threat level was this week raised to "Critical" following the suspected terror attack at Parson's Green tube stop on London's District Line.