The clock tower, home to the bell named Big Ben, was completed in 1859.
Imperial College London’s Prof John Burland, a construction expert, told BBC Radio 4’s Today programme that the lean had “been there for years”.
He added: “We made measurements on it. It was leaning at one in 250 to the vertical, which is just about visible. That’s the break point between looking vertical and looking like a slight lean.
“We’ve known about it for years and it was probably developed at a very early stage because there’s no cracking in the cladding and we think it probably leant while they were building it and before they put the cladding on.”
Although the lean is not quite as severe as that of the Leaning Tower of Pisa, it can be seen by the naked eye.
Prof Burland said: “If you stand in Parliament Square and look towards it, you can just see that it moves very slightly to the left, but I wouldn’t put any political slant on that.”
He insisted that there was no immediate danger, adding: “I calculated that it would take 10,000 years to reach the inclination of the Leaning Tower of Pisa. It’s moving incredibly slowly and always has done so and there really is no immediate danger at all.”