Parliamentary authorities have admitted that the cost of the project, which doubled two years ago, had again risen, from £61.1m to £79.7m.

A breakdown of the overrun is understood to have identified a failed attempt by the Commons’ specialist clock team to restore the timepiece’s mechanism as a significant cause. The mechanism had to be sent off-site, adding almost £2m to the bill.

Other overruns include an extra £2m on scaffolding and £5m to clean up hazardous materials. Contractors’ fees have risen by £2m. Almost double the estimated number of stone repairs - 700 - have been required.

Ian Ailles, director-general of the Commons, said that the task of restoring the 177-year-old, 96m Elizabeth Tower, which houses Big Ben, “had been more complex than we could have anticipated”.

“With a 12m square [130 sq ft] 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,” he told The Times.

“Alongside other issues, such as the impact of often inappropriate conservation methods used by our predecessors, the corrosive levels of pollution in the atmosphere and the discovery of asbestos in unexpected places, we have only now been able to fully understand the full investment required for this project.”

A spokesman for the House of Commons Commission said: “It is very frustrating to learn that the Elizabeth Tower project requires yet more funding, having agreed an extra £32m in 2017.”