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.

Research by Which? found that 87 per cent of customers are unaware that the technology is in use.

The consumer group wrote to 67 cosmetics companies asking them about their use of nanotechnology, the benefits they thought it brought and how they ensured product safety.

Only 17 responded  -  and only eight were willing to provide information about their use of nanotechnology or particles.

Meanwhile, scientists surveyed by the consumer group expressed concerns about the use of carbon fullerenes, which appear in a number of anti-ageing creams.

Nanoparticles are also used to make sunscreens transparent on the skin. But although firms say they offer greater protection against the sun, EU experts have asked for more safety tests to investigate their effect on sunburnt skin.

Which? said: 'Our experts raised particular concerns about potential toxicity of fullerenes if they were able to penetrate the skin. They were also concerned about the widespread use of nano titanium dioxide and zinc oxide in sunscreens, when further research still needs to be done on their safety in nano form.'

Some beauty firms are also using nano silver, which has anti-bacterial properties, in products such as toothpaste. But Which? says there are fears it may be toxic.

Sue Davies, of Which?, said: 'The cosmetics industry needs to stop burying its head in the sand and come clean about how it is using nanotechnology.

'The Government must introduce a compulsory reporting scheme for manufactured nanomaterials so we are all aware  -  and only those that are independently assessed as safe should be allowed to be used in cosmetics.'

Advocates of nanotechnology say the particles could herald many benefits, for instance, delivering medicines and vitamins more effectively.

Professor Dowling said: 'Nanoparticles of a chemical can have different properties to the same chemical in its larger form. And this is why nanoparticles are so exciting.

'However, when cosmetics companies are seeking to exploit these novel qualities in their products they must ensure that their safety testing methods take account of these qualities.'

Those manufacturers who admitted to using nanoparticles said they delivered health benefits.

The Body Shop said: 'They are used for their capacity to neutralise UV light, thus protecting human skin against adverse effects of UV radiation, including skin cancer.'

Boots said it 'does not consider its current use of materials that approach the nano size-range as being of health concern'.

Via Dailymail