A list of shelters has been agreed to cope with a mass evacuation from London if the city is hit by a terror attack.
Selected railways stations will transport people to safety
Council chiefs in Surrey have outlined plans to house and transport tens of thousands of people in an emergency.
Rail operators would lay on special shuttle services to transport people to selected train stations.
From there buses would take evacuees to public buildings such as leisure centres or race courses which would become emergency shelters.
A number of railway stations have been selected to transport people out of the capital.
Surrey's head of emergency planning Jerry Marsh said all the Home Counties were putting together plans in the event of an attack.
"It's a real threat. How likely it is, is very difficult to say but we must be planning for this sort of thing. A lot of contingency planning we hope we never have to put in place."
He said although the plan accommodated people for the first 48 hours of an evacuation from London, there was no suggestion that people would be left to fend for themselves after that time.
The Metropolitan Police, emergency services, transport operators and local authorities announced that a contingency plan was being prepared in May.
At the time government minister Nick Raynsford said there was no need for alarm but "sensible precautions" had to be taken so any area of London could be evacuated.
A spokesman added there was no intelligence an attack was "likely".