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.
More top news
Liberal Democrat members have overwhelmingly voted to cancel Brexit, should their party come to power at the next general election.
The former Tory said his new party would continue to campaign for a second referendum.
The south with enjoy highs of 25C today but the rest of the country will experience mild conditions.