Police have stepped up patrols in London and warned they would make greater use of stop-and-search powers after a third failed car bomb attack in 36 hours.
Senior officers said they were linking the attack on Glasgow Airport with the discovery of two vehicles packed with explosives in the capital on Friday.
Scotland Yard said security plans for London were reviewed after the national terror threat level was raised on Saturday night to "critical", its highest level, meaning an attack is expected "imminently".
Extra officers are being deployed to landmarks, main train stations, Heathrow Airport and London City Airport, the force said.
Police will also make greater use of their power to stop and search people under the Terrorism Act as a "visible deterrent and disruptive tactic".
People attending the memorial concert for Diana, Princess of Wales, at Wembley Stadium are warned to expect delays because of more thorough searches.
About 450 officers will be on duty to police the event, Scotland Yard says.
Two cars loaded with explosives were discovered in London's West End early on Friday morning.
The first, a Mercedes packed with a deadly cargo of petrol, gas canisters and nails, was found by paramedics who were called to the Tiger Tiger nightclub shortly before 2am after a person was taken ill.
The second car, also a Mercedes, was issued with a parking ticket in Cockspur Street, just yards from the first vehicle, at about 2.30am. It was taken to a car pound off Park Lane by Westminster Council at around 3.30am. Workers at the compound were immediately suspicious of the car because it had a strong smell of petrol. But it was not until later when they heard about the news of the first car bomb they contacted authorities.
