The bill to repair Britain's crumbling Houses of Parliament could hit £7.1 billion unless lawmakers move out for up to six years, according to a study published Thursday.
The independent report, commissioned by parliamentary authorities in 2013, found that the famed neo-Gothic building posed major fire risks and was threatened with roof collapses.
The experts, led by Deloitte Real Estate, laid out a range of repair options, from lawmakers staying put while work was carried out to a "full decant", which would see both the House of Commons and House of Lords move to temporary bases.
The first option would take an estimated 32 years and could cost £7.1 billion ($11.3 billion, 9.9 billion euros), while the second would take six years at a cost of around £3.9 billion.
Alternatively, it would cost around £4.4 billion to carry out the works if groups of lawmakers took turns to vacate the premises.
The Grade 1 listed building has leaking roofs that force members of parliament to use waste paper baskets to catch drips when it rains and air pollution has eroded the ornate stonework.
The mechanical and electrical infrastructure is "no longer fit for purpose", and the risk of a "catastrophic failure is increasing", the report said.
Asbestos is a constant hazard and even the famous Big Ben clock tower snapped by tourists from around the world leans 46 centimetres (18 inches) off the vertical.
MPs also complain of cramped working areas and faulty heating.
A parliamentary committee has been set up to decide how and when to carry out the work.
Whatever the decision, full repairs are unlikely to begin before 2020.
The Houses of Parliament, designed by Charles Barry and Augustus Pugin, were finished in 1870 and the building is now a UNESCO World Heritage Site.
If they are forced to move, it is thought most likely that lawmakers would relocate to a nearby spot such as the Queen Elizabeth II Conference Centre, a modern concrete venue near parliament.
The issue is likely to stir controversy between taxpayers who baulk at shelling out for a new home for unpopular politicians and those who believe such an iconic building should be saved.