Bong! Big Ben becoming leaning tower of London, say engineers

London's tower of Big Ben is leaning so much that its tilt can now be seen with the naked eye, civil engineers have discovered.

Surveyors have found that the clock tower at the Palace of Westminster has developed a tilt, which is getting worse every year.

The top of the tower is now almost one-and-a-half feet off the perpendicular – so far off that experts say the tilt is visible to the naked eye.

If the movement was to continue uncorrected, the tower would one day topple. However, MPs can breathe easy: at its current speed it would take some 4,000 years to reach the angle of the Leaning Tower of Pisa, and even longer to hit tipping-point.

In the unlikely event that the tower falls, it will land on MPs' offices over the road in Portcullis House – which may at least offer some compensation to architectural purists unimpressed by the modern building.

Civil engineers believe that the tower - known colloquially as Big Ben after the main bell it houses - is gradually "sinking" or settling into the land on which it is built. But the pattern is uneven, with the sinking occuring more quickly on the north side than the south.

The problem has been blamed on decades of building work that have gone on around the foot of the structure - 315ft (96m) tall, with 11 storeys and 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 a bid to secure it.

Yet a new survey for London Underground and the Parliamentary Estates Department has found that the rate of movement has accellerated in recent years.

The report, completed in 2009 but only just published by the parliamentary authorities, discloses that between November 2002 and August 2003 - the period when MPs staged heated debates on the invasion of Iraq - a mystery "event" caused the tower to lurch, with the clock face moving up to an eighth of an inch (3.3mm) away from the vertical.

The engineers conclude that no single known factor can fully explain the "event". Since 2003, the monitoring instruments suggest the tilt has continued to increase by 0.04in (0.9mm) a year, compared to the long-term average rate of just 0.025in (0.65mm) a year.

The report states: "The overall trend of the data suggests an increased rate of movement which commenced around (August 2003)."

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 (435mm) 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 corridors where ministers and shadow ministers have their offices. The report 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 a flat in the shadow of the tower. However, there have been cracks noted in the area overlooking Speaker's Green, which they have used as a garden.

John Burland, emeritus professor and senior research investigator from Imperial College London who has worked on the Big Ben tower – as well as 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 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."

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Jane Martinson 13 September 2011