US Secretary of State John Kerry has emphatically underlined the evidence America claims points to Assad's regime attacking its own people with chemical weapons on August 21.
He said: "Our intelligence community has carefully reviewed and re-reviewed information regarding this attack, and I will tell you it has done so more than mindful of the Iraq experience - we will not repeat that moment."
More top news
The country's counter-terrorism forces have regained control of the facility.
The commissioner of the Metropolitan Police has said he cannot promise the public an attack will not take place.
The showers are likely to be particularly heavy across northern parts of the country.