An extra £18.6 million is needed to repair the tower which houses Big Ben, parliamentary authorities have been told.
The new costs would see the renovation bill rise by almost a third from £61.1 million to £79.7 million.
The increased bill has been put down to the discovery of asbestos pollution and World War Two damage discovered in the Elizabeth Tower, which houses the famous bell.
A spokesman for the House of Commons Commission said members were “extremely disappointed” by the request for “yet more funding”.
“It is very frustrating to learn that the Elizabeth Tower project requires yet more funding, having agreed an extra £32 million in 2017," he said.
“We have requested more detailed information about the lessons learned from this experience - as well as assurances that more robust estimates are prepared for works of this nature in the future.”
The refurbishment of the 177-year-old structure is set to be completed by late 2021 but Ian Ailles, director general of the House of Commons, said the works are proving “more complex than we could have anticipated”.
New discoveries show asbestos in the belfry, broken glass in the clock dials, extensive use of toxic lead paint and defects in previous work.
Mr Ailles continued: “With a 12m square (130 square feet) footprint and a prime location right in the middle of a busy working Parliament, understanding the full extent of the damage to the Tower was impossible until the scaffolding was up.”
The new budget will be set if it is approved by the accounting officers of the two houses.
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