The threat level set by the security services is now severe after being raised from substantial, amid warnings of a threat from Islamic State (IS) militants.
Raffaello Pantucci of the Royal United Services Institute said the move suggests officials are "very concerned" about the terror threat linked to Iraq and Syria. He said: "This is something that has been building for some time. It seems to have been ratcheted up even more recently.
"There is potential for hundreds of people with knowledge of the battlefield to come back. It all speaks to a very dangerous confluence of events."
Suspected jihadists may be stripped of their passports under new laws.
Prime Minister David Cameron said the number of armed police in public places may rise, but he said people should not be too alarmed.