The UK government had received intelligence based on intercepted communications between militants in the Sinai Peninsula.
However, both Egypt and Russia have said it is too early to draw conclusions.
The UK government suspended all flights to and from the Egyptian resort of Sharm el-Sheikh two days ago.
The move came after it said the incident was likely to be have been caused by terrorism.
Britons stranded in Sharm el-Sheikh since then are due to begin returning home, but will only be allowed to bring hand luggage.
BBC security correspondent Frank Gardner says that although British officials have not ruled out the possibility of a technical fault bringing the plane down, they think that is increasingly unlikely.
Security service investigators suspect someone with access to the aircraft's baggage compartment inserted an explosive device inside or on top of the luggage just before the plane took off, Gardner said.
The Metrojet Airbus A321 was flying from Sharm el-Sheikh to St Petersburg when it came down in Sinai on Saturday, killing all 224 people on board. Most of the victims were Russian.