The International Earth Rotation and Reference Systems Service, based at the Paris Observatory, tells the world every six months whether to add or subtract a second from atomic clocks, the standard for everyday timekeeping.
A leap second is added to Co-ordinated Universal Time (UTC) to keep it in step with solar time - based on the Earth's rotation on itself - to within a second.
Tidal friction causes the Earth's rotation to slow down, which means that solar time tends to drift out of sync with atomic clocks.
If this disparity was not corrected, the "error" could increase to several seconds within a few decades; and, claim astronomers, eventually make some of their software and possibly hardware obsolete.
There have been 22 leap seconds added - and no subtractions - since the first one on 30 June, 1972.
The new leap second will be inserted at the end of the final minute of 2005, giving the familiar "six pip" BBC radio time signal an extra pip before the long pip marking the hour.