Delaying restoration of Britain's crumbling Houses of Parliament could add £420 million (€485 million) a year to the final bill, the country's National Audit Office (NAO) forecast Monday.
Plans to restore the Victorian neo-Gothic edifice, which is part of a UNESCO World Heritage Site featuring Westminster Abbey, stretch back more than a decade.
However works have yet to start owing to concerns over the project's huge costs estimated at tens of billions of pounds, as well as on worries about disruption caused by rehousing MPs and Lords in temporary buildings for many years.
The Houses of Parliament, otherwise referred to as the Palace of Westminster, suffers from "deteriorating mechanical and electrical systems, fire safety issues....(and) asbestos," the NAO confirmed Monday.
In an updated report, the independent spending watchdog noted "that each year of delay corresponds to an additional £320 million (€369 million) to £420 million (€485 million) on top of the overall cost of delivering the programme.
"Parliament currently spends around £1.5 million (€1.7 million) a week on maintenance of the Palace, including major refurbishment projects," the NAO added.
A parliamentary body has proposed various options for the restoration, with one possible outcome showing a phased project lasting between 38-61 years and costing up to £39.2 billion (€45.3 billion).
A "full decant" would see parliament's upper and lower chambers leave the Palace for much of the project, costing up to £15.6 billion (€18 billion) and taking between 19-24 years to complete.
MPs in the lower House of Commons and peers in the House of Lords, must decide on the options, which include one unlikely restoration project forecast to last 84 years, by mid-2030.
Both Houses have additionally been asked to agree to initial restoration works lasting seven years at a cost of up to £3 billion (€3.4 billion).
The Great Clock that towers above the Houses of Parliament, and which carries the famous Big Ben bell, underwent a five-year restoration project through to 2022 at a cost of £80 million (€92 million).
