London's Big Ben is leaning so much that its tilt can now be seen with the naked eye, it emerged today.
Civil engineers have discovered that the clock tower at the Palace of Westminster is no longer ramrod straight and the problem is getting worse every year.
The top of the tower is now almost one-and-a-half feet off the perpendicular and if left uncorrected it would eventually fall.
But before MPs start running scared, at the current rate of movement it would be 4,000 years before it would match the angle of the Leaning Tower of Pisa and even longer to actually topple.
Surveyors believe Big Ben is sinking into the ground because of decades of underground building work since it was built in 1853.
These have included a sewer in the 1860s for the District Line to an underground car park for MPs in the 1970s and the Jubilee Line extension through Westminster in the 1990s.
If Big Ben were to fall it would crash into MPs' offices over the road in Portcullis House. Currently, the clock tower is sinking more quickly on the north side of the 315ft-tall building.
A just published 2009 survey for London Underground and the Parliamentary Estates Department found that the rate of movement has accelerated in recent years.
Engineers cannot explain why the tower's clock face moved up to an eighth of an inch (3.3mm) away from the vertical between November 2002 and August 2003.
Since 2003, monitoring instruments show the tilt has increased 0.04in (0.9mm) a year, compared to the long-term average rate of just 0.025in (0.65mm) a year.
The report, obtained by the Sunday Telegraph, revealed that the tower was now leaning towards the north-west at an angle of 0.26 degrees, meaning the top of the tower is 1ft 5in (435mm) from vertical.
The Leaning Tower of Pisa leans by around four degrees.
Big Ben's movement has caused cracks in the walls of other parts of the House of Commons, including corridors where ministers and shadow ministers have their offices.
John Burland, emeritus professor and senior research investigator from Imperial College London who has worked on Big Ben and the tower in Pisa, told the paper: "The tilt is now just about visible. I have heard tourists there taking photographs saying 'I don't think it is quite vertical" – and they are quite right."
"If it started greater acceleration, we would have to look at doing something but I don't think we need to do anything for a few years yet."