The repair works to the tower which houses Big Ben will cost an extra £18.6m, parliamentary authorities have been told.
This will increase the renovation bill by almost a third, from just over £61m 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.
Members of the House of Commons Commission were "extremely disappointed" by the request for "yet more funding", a spokesman said.
"It is very frustrating to learn that the Elizabeth Tower project requires yet more funding, having agreed an extra £32 million in 2017," they continued.
"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 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.
Ian Ailles, Director General of the House of Commons, said the works are proving "more complex than we could have anticipated".
The new budget will be set if it is approved by the accounting officers of the two houses.
Cover image: Big Ben wrapped in scaffolding on Armistice Day 2019 (Picture: PA).