Clock ticking for leaning Big Ben
Big Ben is leaning to one side and may eventually become unstable - but only in thousands of years, according to a newly revealed report.
The tower leans 0.26 degrees to the north-west, putting it out of alignment by about 0.5m at its highest point.
The lean was discovered when Transport for London commissioned a report, because the extension of the Jubilee Line passes under Parliament.
Experts are unsure what is causing the tower to lean.
But one theory is that the London clay on which the tower was built is drying out.
Mike McCann, keeper of the great clock, told BBC London: "We have been monitoring it since 1999, so we've got some pretty good data.
"Our resident expert believes it will be between 4,000 and 10,000 years before it becomes a problem.
"So it's not significant today, but we do need to keep an eye on it."
The 0.26 degree angle is one 16th of the Leaning Tower of Pisa's tilt.
Mr McCann continued: "There's no real proof what has caused it.
"But it is built on London clay and that can dry out and that can cause movement."
There is no evidence the lean was caused by work on the Underground.
Big Ben is the name of the bell contained within the Clock Tower, according to a spokeswoman at the Houses of Parliament.
But in the recent years the tower, which was called St Stephen's Tower by Victorian journalists, has been known colloquially as Big Ben.