The bongs of London's Big Ben have been mysteriously running fast over the past fortnight, clocksmiths have admitted.
The Great Clock that towers over the British parliament can be out by up to six seconds, with its keepers admitting the cherished national icon is "a little temperamental" at 156 years old.
Over the past two weeks, the early bongs have been upsetting BBC domestic and world radio transmissions that broadcast the hour chimes live.
The Houses of Parliament's three dedicated clocksmiths have tried to rectify the problem, but are somewhat mystified as to why it has swung so far out of step.
"The error started building up and went slightly unnoticed over a weekend," clocksmith Ian Westworth told BBC radio.
"We don't know why it happened. You're talking about a 156-year-old clock; it does have a little fit every now and then. It's a little temperamental.
Clocksmiths regulate the mechanism by stacking heavy old one penny coins on the pendulum, or removing them.
"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," said Mr Westworth.
Initial attempts by the team to correct the mechanism made the clock run slow. "We have been up there most days just getting it right," he said.
"We phone up the speaking clock and at five minutes to the hour, start a stopwatch, go up to the belfry, stand by the bell and the hammer.
"As it strikes the bell we'll stop the stopwatch. We can tell if it's going slightly fast or slow."
Big Ben is the name of the Great Bell at the top of the 96-metre-high Elizabeth Tower, but is often used to refer to the tower itself, which looms over the Houses of Parliament.