The bongs of Big Ben have become “temperamental” and can be out by up to six seconds, Parliament’s clocksmiths have admitted.
BBC Radio 4 broadcasts, which use the sound live, have been interrupted due to the anomaly, causing the bongs to come early for the past fortnight.
The Houses of Parliament’s three clocksmiths have tried to deal with the problem by removing weights from the pendulum.
But they have no idea why it has happened, saying that at 156 years old the Great Clock just has “fits every now and then”.
Ian Westworth, one of the clocksmiths, told Radio 4’s PM programme: “The error started building up and went slightly unnoticed over a weekend.
You can’t just wind the hands forward. You have to make a very gradual change by adding coins to speed the clock up or taking weight off to slow it back down again.”
Initial attempts by the team to correct the mechanism made it run slow. Mr Westworth added: “We have been up there most days just getting it right.”