Flight 7K9268, en route from Sharm el-Sheikh in Egypt to St Petersburg in Russia, took off at 5.51am Cairo time (3.51 UK time). It vanished from radar screens less than half an hour later.
The aircraft, an Airbus A 321 carrying 224 passengers, was operated by Russian airline Kogalymavia. It crashed in a remote area of the Sinai peninsula, and at least 100 people are reported to have lost their lives.
This is the second time in four years that airline Kogalymavia hits the headlines over an air disaster. Back in 2011, three people were killed and 43 more were injured when a Kogalymavia plane caught fire while taxiing for take-off in Russia.
The cause of this crash is unknown at this time. The plane reportedly reached its cruising altitude of 31,000ft and then descended rapidly.
Search and rescue teams are at the scene, and have reported hearing voices coming from the wreckage.