Prime Minister David Cameron on Saturday warned that "a number" of British casualties were expected from a series of attacks in Paris claimed by Islamic State jihadists which killed at least 128 people.
"We must be prepared for a number of British casualties," he said after an emergency cabinet meeting in Downing Street that kept the threat level for Britain at "severe" -- the second highest.
"We will review our plans and make sure we learnt the appropriate lessons," he said. There had been "planning and coordination" behind the Paris attacks, he said.
Cameron suggested the threat posed by the Islamic State group was "evolving".
He condemned the attackers as "brutal and callous murderers", adding: "We will redouble our efforts to wipe out this poisonous, extremist ideology".
Britain held its biggest-ever counter-terror drill in June, involving 1,000 police officers, special forces, ambulance and fire crews in a simulation of a "marauding terrorist attack".
The exercise came shortly after a spree killing on the Tunisian resort of Sousse in which 38 people were killed -- 30 of them Britons.
It also came ahead of the 10th anniversary commemorations of the July 7, 2005 attacks on London's transport system, when four suicide bombers killed 52 people.