The bill to repair London's iconic Big Ben clock tower has increased by millions of pounds after the discovery of World War II bomb damage, pollution and asbestos, Britain's parliament said Thursday.

The cost of renovating the Elizabeth Tower housing the famous clock, which began in 2017, is now set to rise to nearly GBP 80 million (USD 104 million, 95 million euros).

The extensive damage requiring an extra GBP 18.6 million was only revealed once the project team was able to begin intrusive surveys for the first time ever on the 177-year-old structure.

However, the efforts to restore the tower “to its previous splendour” remain on track for completion in late 2021, according to officials.

A spokesperson for the House of Commons Commission, which is overseeing the project, called the spiralling costs “very frustrating”.

Ian Ailles, director general of the House of Commons, said the task of restoring the Elizabeth Tower “had been more complex than we could have anticipated”.

“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,” he added.

The work is to repair the clock faces and mechanism, cracks in the tower's masonry and corrosion in the roof, as well as to restore the edging around the clock faces to its original 19th-century colour.

A lift will also be installed as an alternative to the steps up the tower, and the lights illuminating the clock will be replaced by low-energy LEDs.

Completed in neo-Gothic style in 1856 by parliament architect Augustus Pugin, the tower has suffered from considerable degradation over time, and now even leans 46 centimetres (18 inches) off the vertical.

The renovations have meant Big Ben -- whose chimes feature on British television and radio news bulletins -- has been largely silent since 2017.

That has proved contentious, sparking protests in particular when they did not sound to mark Britain's departure from the European Union last month.