ITV News understands that there were people working on the roof of the National Library of Wales today, when the fire started.
Nobody is thought to have been injured.
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It'll be an unsettled afternoon across Wales, with blustery winds and heavy rain. Gales expected in many areas.
A bronze war memorial which was moved from a fire damaged chapel in Aberystwyth seven years ago is now being returned to its original place.
The compulsory 'scores on the doors' system is helping to drive up standards across Wales, according to the Deputy Health Minister.