The planned restoration of the Palace of Westminster could cost taxpayers more than £3 billion, it has been claimed.
An expert who led an examination of options for renovating the historic building told BBC2's Newsnight that it was "not unreasonable" to think the bill could top £2 billion, but the programme claimed that the "working assumption" of insiders was that it could be a billion more.
Dr Richard Ware was appointed in 2012 as director of a study group into the restoration and renewal of the palace, and later that year produced a report which put the capital cost of the necessary work at around £1.5 billion.
The report found that basic services within the building, like electricity, water and sanitation, were being kept functioning "with increasing difficulty and growing risks", while asbestos was present throughout the palace and original roofs were no longer watertight, leading to extensive damp, leaks and floods.
The present building - home to Houses of Lords since 1847 and the Commons since 1852 - has had no general renovation since repairs of wartime damage in 1945-50, said the 2012 report, adding: "If the palace were not a listed building of the highest heritage value, its owners would probably be advised to demolish and rebuild."
The report considered the options of constructing a new home for Parliament, decanting one or both houses temporarily while Westminster is renovated or attempting to restore the building with MPs and peers working inside - something it warned could take 50 years. And it proposed the establishment of a quango, along the lines of the Olympic Delivery Authority, to oversee the work.
The House of Commons was told last week that £7 million was being spent on a further consultants' report, with MPs due to choose their favoured option in spring 2016 and work not expected to begin in earnest until after 2020.
Dr Ware told Newsnight that if nothing was done, politicians and staff would end up "working in a ruin".
"We're moving backwards, the building is getting older, faster than we can deal with it", he said. "The building is on borrowed time, and if we don't act soon we won't have a choice."
Asked if the cost would be more than £2 billion, Dr Ware said it was "not unreasonable to think it will be of that order".
But Newsnight said it had been told that the working assumption was that the cost could reach £3 billion over many years. It quoted an unnamed source familiar with the project as saying: "I'd be surprised if it stayed at that'.