Thousands of tourists are stranded in Egypt after mounting evidence a Russian airliner was downed by a bomb led to all flights between the UK and Sharm el-Sheikh being suspended.
Emergency screening is being put in place at the popular resort's airport to allow British nationals to be flown home after an inspection by UK experts resulted in all flights in and out of the popular Red Sea resort being halted.
The team was sent in by the Government after fresh intelligence suggested there was a "significant possibility" that a bomb had been placed on a Russian aircraft that crashed 20 minutes after taking off, killing all 224 on board.
A meeting of the emergency Cobra committee, chaired by the Prime Minister, decided to issue a warning against "all but essential" travel through the airport - effectively barring flights to and from the UK.
The move - angrily denounced by Egypt's foreign minister as a "premature and unwarranted" step that would smash its tourist industry - was announced as the country's president Abdel Fatah al-Sisi arrived in the UK for a scheduled visit.
He is due to hold talks with Mr Cameron at 10 Downing Street later today.
Foreign Secretary Philip Hammond said he recognised the potential "huge negative impact" but insisted ministers had no choice but to act on intelligence "which we believe represents a threat to British nationals".
He said his angry counterpart Sameh Shoukry "hasn't seen all the information that we have".
"While we regard the Egyptians as very important partners - and want to work with them not just on airport security but on all aspects of the development of their economy and the building of Anglo-Egyptian relations - when we see something which we believe represents a threat to British nationals we have to act on it and the other consequences have to be dealt with."
The decision was taken "very reluctantly", he said, and praised Egyptian authorities for "moving heaven and earth to meet our demands on the ground".
Officials are working with airlines to find a way to safely return people to the UK - either on their scheduled flight or earlier if they wish - although no flights are expected to leave before at least Friday.
The change in official Foreign Office travel advice applied only to the airport, with Sharm itself - where there are believed to be 20,000 Britons at present including a small proportion of ex-pats - still considered safe.
Mr Hammond apologised for the "immense disruption and inconvenience" caused - including to people who had been forced to return to hotels from the airport.
"I also recognise the immense impact that this will have on the Egyptian economy. But we have to put the safety and security of British nationals above all other considerations.
"When we are in possession of information we will not hesitate to act on it in order to protect that security and we will take whatever criticisms we receive."
The US also said initial intelligence suggested the plane - an Airbus operated by Metrojet and bound for St Petersburg carrying mostly Russian tourists - had been blown up by terrorists.
Cairo has sought to dismiss claims that the crash was the work of Islamist terrorists, such as the self-styled Islamic State (IS) and complained that the UK had acted before investigations were complete.
Mr Hammond said the Cobra meeting of senior ministers and security officials "reviewed all the information that we have available from a range of sources" about the plane crash.
"As a result of that review we have concluded there is a significant possibility that that crash was caused by an explosive device on board the aircraft," he said.
Downing Street said the information that prompted the move included "some that has recently come to light".
It was reported intercepted communications played some part in a preliminary US finding that a bomb had been planted on the aircraft by IS's Sinai affiliate - though there had been no formal conclusions drawn.