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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Boris Johnson and Jeremy Corbyn debate the issues of the General Election in Salford.
Boris Johnson and Labour leader Jeremy Corbyn go head-to-head in the first televised debate of the General Election campaign.
Dry for many tonight with some frost and fog.