Scotland Yard has warned that al Qaeda terrorists are plotting to use the cover of huge anti-war protests against George Bush next week.

Senior policemen said that concerns over an attack on the president are behind the biggest security operation Britain has seen for any visiting head of state.

However, the centre of London will not be sealed off during the visit, from November 18 to 21. "Some roads will be closed for a time for operational reasons but the Mall and Whitehall will remain open," a police source said.