The tower, colloquially known as Big Ben, has been slightly unbalanced since the 19th century. But documents drawn up by engineers note that it underwent a new and so-far unexplained shift in 2003.

The documents were recently posted to parliament’s website in response to a freedom of information request from the Sunday Telegraph, which quoted engineering expert John Burland as saying the tilt was just about visible to the naked eye.

But the man charged with the tower’s clock, Mike McCann, said Monday that the tilt was “very, very minor.”