The most famous clock in the world is wrong: 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 messed up 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.

"Imagine running your car for 24 hours a day, 365 days a year for the last 156 years."

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," Mr Westworth said.

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.

"Traditionally we have to go up three times a week to wind the clock.

"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 the moniker is often used to refer to the tower itself, which looms over the Houses of Parliament.

The 13.7-tonne bell, with its distinctive bongs, sounds out the hours over central London, while different chimes mark every quarter hour.

There are two theories as to how the bell came to be known as Big Ben.

The most likely explanation is that it was named after Benjamin Hall, the engineer whose name is inscribed on the bell, but some believe it is named after Ben Caunt, a champion heavyweight boxer of the 1850s.

AFP