Laser pens sold online for less than £20 could cause "catastrophic" eye damage and permanent loss of vision, scientists have found.
The study of 'pointer' laser pens in Australia found that half exceeded safety standards intended to prevent super-strong beams being sold to the general public.
The research has prompted alarm with the Royal National Institute for the Blind (RNIB) calling for stronger regulations to prevent accidents caused by the device.
It comes after repeated accidents involving lasers including a number of cases in which pilots have been targeted by the beams.
One pilot was said to have suffered permanent eye damage after he was hit with a "military strength" laser while landing at Heathrow Airport in 2015.
The RNIB said that it "extremely concerning" that people could be buying lasers without understanding their potential for harm.
The Australian research looked at eight laser pens bought legally online for less then $30 Australian dollars - the equivalent of £17.50.
They found that all four of the stronger green-coloured pens failed Australian safety safety standards, with power ratings between 51 and 127 times higher than the one milliwatt legal limit.
"At that upper level, the beam would cause catastrophic retinal damage," said the study's lead scientist Dr Kate Fox, from RMIT University in Melbourne.
All four red lights, which are less strong, were within the safety standards.
The UK has similar health standards on lasers to Australia.
Public Health England says that lasers with a maximum power of more than one milliwatt and a beam wavelength of over 700 nanometres are "too powerful for general use" and pose an "unacceptable risk" in the hands of consumers.
However, there are fears that powerful laser pointers may still be available online from foreign suppliers.
The Australian researchers also warned that imported laser pointers were often poorly made, and manufacturers were tempted to skip installing infrared-blocking safety filters to hold down costs.
The findings were presented at the IEEE Engineers in Medicine and Biology Society conference in Orlando, Florida.