George Bush Sr has been moved into intensive care after suffering "a series of setbacks that include a persistent fever," according to a family spokesman.
Doctors are "cautiously optimistic" about the 88-year-old former president's health, he added.
Video report by Debi Edward.
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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.