The cost of repairing the Elizabeth Tower, which houses the famous Big Ben bell, has risen by £18.6m following the discovery of bomb damage and asbestos.
The need for more money was only discovered during a survey of the 177-year-old structure in central London.
The House of Commons Commission said it was "extremely disappointed" that the cost had risen to £79.7m.
The new budget will have to be approved by the accounting officers of the Houses of Parliament.
