Don't set your watch by Big Ben.

Officials say the famous clock at Britain's Parliament — used by Londoners for decades to check the time — has recently been slow by as much as six seconds.

The 156-year-old clock chimes every 15 minutes and emits deep bongs to mark the hour. Its inaccuracy was noticed by staff at BBC radio, which broadcasts the bongs live at 6 p.m.

Ian Westworth, one of Parliament's three clocksmiths, told the BBC the clock had become "a little temperamental" with age.

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

Parliamentary officials said Tuesday that mechanics had corrected the clock to within "normal parameters" — within 2 seconds of the right time. They will continue to adjust it by placing pennies on the pendulum — or removing them — to fine-tune its speed.

Parliament's neo-Gothic clock tower is one of London's most recognizable landmarks, and its bell is one of London's characteristic sounds. The famous bongs echoed through the city without fail through World War II bombing raids, though they have been silenced a few times for repairs, and by accident or the weather.

Although the clock tower is popularly known as Big Ben, the name actually refers to the 13 ½- ton Great Bell inside. The tower was renamed the Elizabeth Tower in 2012 to mark Queen Elizabeth II's 60 years on the throne.