The taxpayer faces a bill of up to £7.1 billion to stop the Palace of Westminster falling down unless MPs and peers agree to move out, according to a report.
A study by independent consultants has highlighted the appalling state of Parliament - with major fire risks and the threat of roof collapses.
It proposes a series of options, from politicians staying in the famous building to a “full decant” that would see both chambers emptied and moved to temporary accommodation.
If MPs and peers remain, the necessary work will take an estimated 32 years and could cost £5.7 billion - with the possibility the figure could hit £7.1 billion.
A mid-option would see MPs and peers vacate the chambers in turn, at a cost of around £4.4 billion. Fully leaving the heritage site while it is renovated into a state-of-the art building with mod cons would take six years and cost around £3.9 billion.
A joint committee of MPs and peers will now be formed to consider the choices.
The Independent Options Appraisal (IOA) was commissioned by the parliamentary authorities in 2013 from an independent team of external experts led by Deloitte Real Estate. The study itself and associated investigations have cost around £8 million.
A statement issued by both Houses said: “The restoration and renewal of the Palace of Westminster will be a major challenge facing Parliament in the coming years and is certain to be a matter of public interest.
“The process to establish a Joint Committee, which will make recommendations to both Houses on how to proceed, is already under way. It will be for the Joint Committee to decide how best to carry out its task.”
A full restoration programme is unlikely to start before 2020.
The 250-page report points out that there have only been ad hoc works on the Victorian building over the past six decades.
As a result, the mechanical and electrical infrastructure is “no longer fit for purpose”, and the risk of a “catastrophic failure is increasing”.
Recent examples of issues include a burst pipe flooding the Committee Room Corridor, part of the ceiling in the Lords Chamber falling on to the benches below, and fears over asbestos.
The paper says “fundamental renovation can no longer be avoided” to protect the Palace - a key part of the “UK brand” - and provide safe and “decent accommodation” for around 2,000 people who work on the site.
If MPs and peers stay in the building and the bare minimum of renovation is carried out around them, costs will be significantly higher.
The project would take between 25 and 40 years, with 32 years being the most likely.
There will also be higher operating expenses during the period, as the Palace will have to wait longer for improvements such as to make it more energy-efficient.
The other options involve a partial or full move out of the Palace, which would dramatically reduce the bill and allow work to be completed much more quickly.
A “rolling” decant, where Lords and MPs vacate their chambers in turn, would mean renovations cost between £3.6 billion and £4.8 billion - with the central estimate £3.9 billion. The project would take around 11 years, according to the report.
A full decant from the Palace would cost £3 billion to £4.3 billion - with the most likely figure £3.5 billion. The most likely timescale in that scenario would be six years.
Prime Minister David Cameron said the issue was “predominantly” for the parliamentary authorities.
But he added: “In all these things we have to be cost effective.”