UK investigators are working under the assumption a bomb was placed on board a Russian airliner which crashed in Egypt.
According to reports from the BBC, UK government officials suspect a device was placed in the hold shortly before take-off.
The UK government has suspended all flights to and from the Egyptian resort of Sharm el-Sheikh as a result.
The working theory is based on intercepted communications between militants in the Sinai Peninsula.
The Metrojet Airbus A321 was flying from Sharm el-Sheikh to St Petersburg when it came down in Sinai on Saturday.
All 224 people on board, mainly Russian nationals, were killed.
However, both Egypt and Russia have said it is too early to draw conclusions about a potential bomb bringing down the plane.
Tens of thousands of Britons remain stranded in Sharm el-Sheikh, with ambitions to begin returning them today.
Under updated security protocols they will only be allowed to bring hand luggage.