The celestial event will last for approximately 5.5 hours, meaning there is plenty of time to catch the rare event.
Viewers in certain areas, such as the US West Coast will not be able to see it until the Sun is visible in the sky.
Mercury will be as close as it is going to get to the centre of the Sun at around 3.20pm GMT (8.20am PT).
The transit will still be underway as Solar System’s star sets in the UK an hour later.
Unlike a transit of Venus, where the planet appears large enough to be seen with the naked eye, Mercury is so small that stargazers will require a telescope or powerful binoculars fitted with a Sun filter.