While buying glasses online can be cheaper than buying from a high street optician, Which? is warning consumers that they may be at risk of ending up with poor quality and potentially dangerous eyewear.

The consumer champion posed as three different customers to order one pair of low prescription single-vision glasses, one pair of high prescription single-vision glasses, and one pair of varifocals from nine online retailers.

Which? bought frames from the lower end of the price spectrum (up to £50 for single vision and up to £120 for varifocals) as price is one of the key reasons people choose to buy online. It then worked with two opticians to measure the accuracy of the glasses received, and assess whether they had been made according to British standards.

Seven of the 26 (27 per cent) pairs of glasses ordered from online retailers failed the Which? tests, either because the actual measurements were too far off what it had supplied and they did not conform to British Standards, or the lenses were loose and could fall out or be easily rotated.

Two of the failed pairs were from Fashion Eyewear and two were from Goggles4U, with Spex4Less, Select Specs and Direct Sight each having one pair that failed.

Almost half (11 out of 26) pairs of glasses ordered were criticised for their poor build quality – eight pairs had poor-quality lenses that were scratched, loose, warped or positioned badly, two pairs had issues with nose-pad positioning, and two had loose arms.

For higher strength prescriptions, opticians recommend that you order high-index, thinner lenses. However, Direct Sight, Fashion Eyewear, Goggles4U and Spex4Less failed to make this clear to customers, producing the higher prescription pair of spectacles with standard lenses that were considered to be much too thick.

In response, Spex4Less said if customers found their lenses too thick, it could thin them out for a fee. Direct Sight said that lens thickness would not compromise the visual acuity of any pair of glasses.

While all nine pairs of glasses Which? ordered with a simple prescription passed the opticians’ checks, this was not the case for glasses with varifocal lenses. Seven out of the nine pairs of varifocals caused concern because no height measurements were taken.

The expert opticians said this could be ‘very unsafe,’ as badly positioned varifocal lenses could cause the wearer to experience vision distortion, which may increase the risk of falling and could be dangerous when driving.

In response, Glasses Direct, Select Specs and Specky Four Eyes explained that they accept returns for any reason, including if the glasses are out of tolerance.

Glasses Direct and Select Specs told Which? that their non-tolerance rates for varifocals were very low, and Glasses Direct says it had conducted research about optimum height measurements for varifocals. Mister Spex explained that it offers a varifocal fitting photo.

Which? has reported its findings to the General Optical Council and is calling for online retailers to improve their standards.

Natalie Hitchins, Which? head of home products and services, said: “Ordering online might seem like a convenient and cheaper way to buy glasses, but we’re warning shoppers to be wary.

“While simple prescriptions are less risky, our research shows that complex glasses, such as varifocals, might not meet the standards we would expect – potentially leaving you with substandard specs.”

Top tips for buying glasses from online retailers

Take care when entering your details, as there’s no automatic right to a refund if you provide an incorrect prescription or have glasses made to your specification (unless they are actually faulty).

Some sites, such as Glasses Direct and Mister Spex, offer a free try-at-home service. This could be a good way to make up for the lack of in-person fitting advice, and you can take some time to decide what fit works for you.

If you’ve got a relatively simple prescription, the Which? investigation suggests you shouldn’t run into too much trouble ordering online.