Pisa expert says Big Ben leaning

Monitoring instruments suggest the tower's tilt has increased by about a centimetre a year since 2003. Photo / Thinkstock

Whether the British parliament is leaning to the left or right depends on your point of view but Big Ben is officially tilting.

And surveyors say it is getting worse.

A new report has found the top of the famous clock tower, which stands above the parliament in central London, is now just under half a metre off the perpendicular.

That is so far off that experts believe the tilt is visible to the naked eye.

"The tilt is now just about visible," said John Burland, a senior research investigator from Imperial College London who has worked on Big Ben and the Leaning Tower of Pisa.

"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," he told the Sunday Telegraph.

"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."

Civil engineers believe the tower is sinking more quickly on the north side than the south side of the Palace of Westminster.

Monitoring instruments have suggested the tilt has increased by about a centimetre a year since 2003, about 40 per cent faster than the long-term average.

The tower is now leaning towards the northwest at an angle of 0.26 degrees, meaning the top of the tower is 43.5cm from vertical.

It would take another 4000 years or so for it to match the angle of the Leaning Tower of Pisa, which leans by about four degrees.

The problem has been blamed on decades of building work around the foot of the 96-metre, 11-storey structure since completion in 1858.

These have ranged from a sewer built for a tube line and an underground car park for MPs in the 1970s.

- AAP