An investigation by Which? found nine firms in the UK selling frames with shoddy lenses and bad measurements, with 27% of glasses ordered from nine online stores failing quality tests.
Measurements were found to be incorrect, and some lenses were loose and could fall out or be easily rotated.
Experts said some glasses were unsafe, since they could distort vision leading to a fall or dangerous driving.
Natalie Hitchins, of Which?, said: “Ordering online might seem like a convenient and cheaper way to buy glasses, but we’re warning shoppers to be wary.”