British landmark Big Ben is leaning to such an extent that the tilt can now be clocked with the naked eye, according to a report commissioned by London Underground and the Parliamentary Estates Department.

The 96 metre (yards) high clock tower of the Houses of Parliament -- known colloquially as Big Ben, the name of the great bell it houses -- is sinking unevenly into the ground, causing it to lean towards the northwest.

"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," emeritus professor and senior research investigator at Imperial College, London, John Burland, told the Sunday Telegraph.

The level of the tilt has accelerated since 2003, increasing to 0.9 mm a year, compared to the long-term average rate of 0.65 mm a year, the report revealed.

These levels are not considered to be unsafe.

"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," Burland said.
Years of underground developments have contributed to the clock tower's tilt, according to the report.

This includes the construction of an underground car park in the early 70s and an extension of the London Underground Jubilee Line, as well as changes in ground conditions.

The tilt has resulted in the formation of cracks in the walls and ceilings of parts of the House of Commons, including the Minister's Wing.

The Palace of Westminster, also known as the Houses of Parliament, is the site of Britain's House of Lords and the House of Commons.

The construction of the great clock tower was completed in 1858. (Source: Reuters)