London's Big Ben appears to have become "temperamental" in recent weeks.
The 156-year-old clock has been out by up to six seconds recently, and as a result has caused interruption on Radio 4, which broadcasts the clock's chimes live, the BBC said.
Ian Westworth, one of parliament's three dedicated clocksmiths, speaking on BBC Radio 4, said the error had gone unnoticed over a weekend and had built up as a result.
He said it was unclear how the error had occurred but, given Big Ben's ages, "it does have a little fit every now and then. It's a little temperamental".
"Imagine running your car for 24 hours a day, 365 days a year for the last 156 years," he said,
The clocksmiths can control Big Ben by adding or removing weights from the pendulum.
"You can't just wind the hands forward," said Mr Westworth.
"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.
(Additional reporting by agencies)
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