HULL History Centre has been forced to close because of a burst pipe.
Water was discovered in the ground-floor library when staff turned up for work yesterday morning, but Hull Council said it did not think any documents or records had been damaged or were at risk.
It could not say when the centre was due to reopen.
The £10.7m facility in Worship Street was officially opened in June 2010 by broadcaster and historian Dan Snow, and is believed to be the first in the UK to house the archives of a city and its university under one roof.
Its oldest document is an 11th-century deed confirming an exchange of land in Bayeux, France. Other significant records and documents include Hull’s Royal Charter, giving the town its autonomy in 1299.
Written in Latin by a chancery clerk in Westminster, the single parchment carries the Royal Seal of Edward I and marks Hull’s emergence as a nationally important port. It grants certain rights and privileges of self-government, including the right to hold court cases, raise revenue, set by-laws and hold markets.