Hundreds of troops have been drafted in to protect Heathrow Airport and other key sites around London amid fears of an an attack by al-Qaeda in the coming days.
Police said the measure was precautionary and designed to coincide with the end of the Muslim festival Eid al-Adha, being celebrated across Britain this week.
"From time to time, it is necessary to raise levels of security activity," the police said in a statement. "We think it is prudent to do so now".
"The current strengthening of security is precautionary and is related to action being taken in other countries and the possibility that the end of the religious festival of Eid may erroneously be used by al-Qaeda and associated networks to mount attacks".
They said heightened security would be most visible at London's biggest airport, Heathrow, and was connected to "a potential threat to the capital".
This is the first time the army has been posted at Heathrow since 1994, when the IRA tried to launch a mortar attack on the airport.
Some 450 troops are expected to guard the airport, with other soldiers working alongside police in central London.