People should not look directly at the sun during a partial eclipse on Tuesday, the government's Chief Medical Officer has warned.
It can lead to permanent damage to eyesight or even blindness, said Dame Sally Davies.
Observing the eclipse through a telescope, binoculars, sunglasses, photographic film or camera is not safe, said The Department of Health.
The safest way is via the television or live webcasts, it added.
Dame Sally Davies said: "Under no circumstances should people look directly at the sun during a partial eclipse.
"The risks of doing so are very real and could lead to irreversible damage to eyesight and even blindness.
"Children are particularly vulnerable as they may be tempted to take a peek.
"We would urge parents to explain the danger to their children.
"We would not wish to see another case like the young boy who lost his central vision back in October 2005 through looking directly at a partial eclipse in his school playground."
The partial eclipse of 4 January will be visible in much of the UK between 0800 and 0930 GMT.
The moon will pass across the front of the sun but not cover it completely, creating a partial eclipse.
The amount of sun obscured in the middle of the eclipse varies from around 75% in London to barely 40% in Glasgow.