An extra £18.6m 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 £61.1m to £79.7m.
The increase has been put down to the discovery of asbestos, pollution and the discovery of extensive Second World War bomb damage in the Elizabeth Tower, which is home to the capital’s famous bell.
But 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.
The full scale of the work needed to complete the refurbishment by the late 2021 deadline only became clear when the renovation team began the first ever “intrusive surveys” on the 177-year-old structure, officials said.
The discoveries included asbestos in the belfry, broken glass in the clock dials, extensive use of toxic lead paint and defects in previous work, the House of Commons and House of Lords Commissions were told.