A specific piece of 'signals intelligence' - such as eavesdropped conversation - picked up at the GCHQ listening station convinced Prime Minister David Cameron that a Russian plane which crashed in Egypt was brought down by a bomb, ITV News understands.
The intelligence led Mr Cameron and his advisers to conclude security at Sharm el-Sheikh airport was breached, with the government suspending flights from the Egyptian resort to the UK on Wednesday and Thursday while security at the airport was reviewed.
Flights to repatriate British tourists from Sharm el-Sheikh will begin on Friday following agreement with the Egyptian authorities on a "package of additional security measures", Downing Street said on Thursday.
Specific signals intelligence convinced PM that bomb was on Russian plane, ITV News learns
But clearly, not universally compelling. US says it hasn't determined the cause. Egypt unconvinced.
Terrorists would be able to bypass the wand-like devices being used to scan luggage in Sharm el-Sheikh, an expert has claimed.
Intelligence showed a "significant possibility" that the crashed Russian passenger jet was brought down by an explosion, ITV News was told.
As flights from Sharm el-Sheikh to the UK are delayed over bomb fears, we review what is known about the Egyptian plane crash tragedy.