Large clouds of black smoke billowed after two disused water towers near the National Archives in Kew, London was said to be on fire.
The main building, which was open to the public, was evacuated after the blaze broke out, a National Archives spokeswoman said.
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The Democratic presidential candidate may also have shown his cards on his choice of running mate.
The US president also shared a post on Twitter accusing Dr Anthony Fauci of misleading the public over hydroxychloroquine.
Fears over an impending second wave of coronavirus dominates Wednesday’s front pages.