Big Ben becoming 'the leaning tower of London'

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 11/2ft 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 some 4,000 years for the tower to reach the angle of the Leaning Tower of Pisa, and even longer to hit tipping point.

In the unlikely event that the tower did fall, it would land on MPs' offices over the road 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 - 315ft tall, 11 storeys, 393 stairs - 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 1970s.

When the Jubilee Line was extended through Westminster in the late 1990s, 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 report says: "The overall trend of the data suggests an increased rate of movement."

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. The Leaning Tower of Pisa, by contrast, leans by around four degrees.

Nevertheless, London's leaning clock tower is already causing cracks in the walls of other parts of the House of Commons.

The report identifies areas affected, including one where ministers and shadow ministers have their offices, and says this should be monitored more closely.

There is no mention of whether the damage has spread to the home of John Bercow, the Speaker, and his wife Sally, who live in the shadow of the tower. But there are cracks in the area overlooking Speaker's Green.

John Burland, emeritus professor and senior research investigator at Imperial College London, who has worked on both 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. I've heard tourists there taking photos saying, 'I don't think it is quite vertical' - and they are 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."