The Palace of Westminster needs to be evacuated for multibillion-pound emergency repairs if it is to avoid the increasing risk of being ravaged by fire or swamped in a sewerage flood, MPs have been warned.
Labour MP Chris Bryant, who is joint spokesman for the Committee on Restoration and Renewal, made the dire predictions as he argued against colleagues who want the work to take place around them.
But even the fast-track repair option would take an estimated six years, and cost taxpayers some £3.5 billion.
The chairman of the Commons Treasury Committee Andrew Tyrie said at the weekend that insufficient evidence had been produced to justify the restoration plan.
Yet as the Press Association was given a rare tour of the warren of subterranean corridors which make up the palace's underbelly, the scale of the problem was obvious as bird's nest-like bunches of telephone wires dangled precariously from the walls.
A mass of 252 miles of electrical cables scrambled along the ceilings as they vied for space with huge gas pipes and the steam central hearing system which sends temperatures soaring in hotspots across the cramped underground gangways.
Mr Bryant noted it was "ironic" that one of the "catastrophic failures" that risked endangering the building could come from a sewerage flood from the Victorian drainage system.
"The main one which we have suffered historically is fire. Some kind of water flood, either cascading down, or cascading up through from the drains is another possibility.
"The sewerage system of this building lies at a lower level than the city drain, as it were, and so all the drainage from every single toilet and kitchen, and so on in the palace, goes down one big drain to the far end underneath the Speaker's garden where it goes into a slightly beautiful contraption that was put in in the 1880s, and then is effectively spat into the main drain.
"Now, if that were to break down, and it's 130 years old now, then that would close the building. End of. Until such time as it is mended."
Fire remains the biggest fear with the largely open plan layout of the underground labyrinth of corridors making any blaze harder to control.
Asbestos remains a concern, and more than 50 blaze-related incidents have occurred since 2008.
Power outages can also be provoked when one of the hordes of rats and mice that infest the palace gets electrocuted on a cable.
The bulk of the money in a full-scale restoration programme would go on replacing the antiquated electrical and mechanical systems underneath the palace, Mr Bryant said.
MPs had been expected to vote on the recommendations of the Restoration Committee this month but Mr Bryant, who favours full evacuation of the Commons and Lords to other sites from 2023, fears the Government is delaying the debate.
He believes ministers want to keep the parliamentary timetable flexible to ensure any emergency Brexit legislation can be dealt with in the run-up to PM Theresa May's triggering of the Article 50 withdrawal negotiations by the end of March.
A 2012 report into the state of decay declared ominously: "If the palace were not a listed building of the highest heritage value, its owners would probably be advised to demolish and rebuild."
Some estimates have concluded that doing the major repairs while both Houses of Parliament stayed put could see the work stretched out to 32 years at a cost of £5.7 billion.
While the biggest dangers to the palace lurk below, it is also under attack from the air by the humble pigeon, as their droppings are corrosive to the brickwork.
But the palace, which is home to 1,180 rooms, 126 staircases, and two miles of corridor, is fighting back with the aid of its own hawk master.
The hawks, Angel, 11, and his younger brother Dennison, 7, patrol the palace skies ready to wage war on any feral pigeon or gull they sight.
Despite its feeling of perpetuity, most of the Palace of Westminster is much younger than the White House and rose from the ashes of the great fire of 1834 that wiped out much of the original medieval building.
Elder statesmen of the day and former prime minister the Iron Duke of Wellington insisted the new palace must rise from the old as he strongly opposed the notion parliament would be rebuilt on land next to Buckingham Palace in what is now Green Park.
Wellington is said to have insisted it must remain on the river bank so "the mob" could never fully surround it, and MPs and peers would always have an escape route via the Thames.
But the mob, now more politely known as public opinion, could well be unhappy at the scale of the bill needed to keep his dream alive.
As we emerged from the bowels of the palace, where the hawks circle high above Big Ben, while the drains beneath threaten to overflow, and the 6,969 fire detectors keep watch, the great bell's ominous tomes boomed out across London, but beware Westminster - lest they toll for thee.
Conservative MP Shailesh Vara said MPs should stay in the Palace while restoration work takes place.
"Following the EU referendum result, at a time when we want to make new friends abroad and secure favourable trade agreements, we should be making the most of this iconic building that is Parliament," Mr Vara told BBC2's Daily Politics.
"At this crucial time, it is absurd that we should be seeking to sell UK plc from a temporary building in the courtyard of the Department of Health. The notion we should be leaving the Palace of Westminster for the convenience of the builders is simply wrong.
"This is an eight-acre site. There is plenty of room here for us to stay while the work is carried on around us."