British stargazers may think "they don't need the same protection as they would do during a total eclipse" and could suffer long-term retinal damage, the College of Optometrists said.
On Monday, just before sunset, the moon will appear to take a "bite" out of the sun in a phenomenon lasting roughly 40 minutes.
The leading optometry body has offered guidance to Britons wanting to watch the lunar event safely.
- Do not look directly at the sun even if wearing sunglasses, which do not protect eyes enough;
- Watching through telescopes, binoculars and cameras is risky and should be avoided;
- TV or webcam broadcasts are a "reliable and safe alternative";
- You can watch directly with specially designed solar filter glasses (with an appropriate CE mark);
- Use the "pinhole projection method". This involves putting a hole in a piece of cardboard, and holding it up - with your back to the sun - so the sun's image is projected on to another piece of paper or card.
Clinical adviser Daniel Hardiman-McCartney, of the College of Optometrists, said: "While there will be a full solar eclipse in the US, we will only have a partial eclipse in the UK, and in some ways this can be more dangerous as many people may feel that they don't need the same protection as they would do during a total eclipse.
"You should never look directly at the sun during a total or partial eclipse. This is because the radiation emitted by the sun is so powerful it may cause long-term harm to the retina."
Gloomy skies are set to stop most Britons seeing the partial eclipse, forecasters have said.
On British shores, only people in south-west England and South Wales are expected to have any chance of witnessing the moment through a break in the cloud.
But, the movement of the moon between the Earth and sun will produce a much more dramatic event in the US, where a total eclipse will turn day to night for two minutes.
The guidance was reiterated by Nasa, which warned: "It is never safe to look directly at the sun's rays - even if the sun is partly obscured."
The space agency is providing several live-streams of the lunar event through Facebook, Twitter and YouTube.