Tiny particles that may be toxic are being used in beauty creams without proper safety testing, a consumer group has warned.
Nanoparticles, which are 80 times thinner than a human hair, are used by firms including Boots, The Body Shop, Avon, Nivea and Unilever.
Some manufacturers believe the technology can deliver the benefits of products in a more effective way.
But critics say the size of the particles may allow them to permeate protective barriers in the body, such as those surrounding the brain or a developing baby in the womb.
Their scale also changes the way they interact with other cells, which might lead to unforeseen toxic effects.
Following a report which raised concerns that some firms are not declaring their use of nanoparticles, Which? has called for the so-called 'grey goo' technology to be banned unless it is proved to be safe.
Its concerns have been backed by Professor Dame Ann Dowling, of the Royal Society, the UK's independent science academy.
She has condemned the beauty industry for failing to provide information on safety testing and use of the controversial ingredients.