The chimes of Big Ben were silenced for over an hour on Friday night but the cause remains a mystery .
The Palace of Westminster's 147-year-old clock, considered to be one of the most reliable in the world,
stopped at 10:07 pm .
The minute hand started to move again slowly but stalled a second time at 10:20 pm where it is reported to have remained for 90 minutes before the hands were reset .
It was thought the hot weather may have been to blame .
Temperatures in London reached a high of 31.8C (90F) yesterday
and forecasters declared it the hottest May day since 1953 .
But an engineer at the palace said the cause was unclear .
He said, "We've been told there was a minor glitch but then it was started up again."