There's something crooked at Westminster
Beneath it, governments have veered from Left to Right, but the tower of Big Ben has always seemed resolutely straight -- until now.
Surveyors have found that it has developed a tilt, which is getting worse every year.
The top of the clock tower at the Palace of Westminster is now almost 1 1/2ft off the perpendicular -- so far off that experts say it is visible to the naked eye.
If the tower's movement were to continue uncorrected, it would one day topple over.
But Westminster's politicians can breathe easy; at the current rate it would take 4,000 years for the tower to reach the angle of the Leaning Tower of Pisa, and even longer to hit its tipping point.
In the unlikely event that the tower did fall, it would land on MPs' offices in Portcullis House, which may please architectural traditionalists unimpressed by the modern building.
Civil engineers believe that the clock tower -- known as Big Ben after the main bell within it -- is gradually "sinking" into the land. But the pattern is uneven, with sinking occurring more quickly on the north side than the south.
The problem has been blamed on decades of building work around the foot of the edifice since it was completed in 1858.
These have ranged from a sewer built in the 1860s, to the District Line the following decade, and an underground car park for MPs in the Seventies. When the Jubilee Line was extended through Westminster in the late-Nineties, special techniques were used to create a concrete barrier under the tower, in an attempt to secure it. Yet a new survey for London Underground and the Parliamentary Estates Department has found that the rate of movement -- which had been steady for many years -- has now accelerated.
Completed in 2009, but only just published, the research finds that between November 2002 and August 2003 -- a period when MPs debated heatedly on Iraq -- a mystery "event" caused the tower to lurch, with the clock face moving up to an eighth of an inch away from the vertical.
The engineers conclude that no single known factor can fully explain the "event". Since 2003, the tilt has continued to increase by 0.04in (0.9mm) a year, around 40 per cent faster than prior to that year.
The tower is now leaning towards the north-west at an angle of 0.26 degrees, meaning the top of the tower is 1ft 5in from vertical. The report says this is within safe limits.
Nevertheless, the leaning is already causing cracks in the walls of other parts of the House of Commons.
John Burland, emeritus professor and senior research investigator at Imperial College London, who has worked on the Big Ben tower and the one in Pisa, said: "The tilt is now just about visible. You can see it if you stand on Parliament Square and look east, towards the river."