Microplastics are produced by wear and tear from products such as polyurethane varnish, car tyres and wet wipes.
(By Thomas Moore, science and medical correspondent)
The World Health Organisation has called for urgent research on the risks of microplastic to human health after confirming that tiny fragments are found in drinking water.
In its first assessment of plastic pollution the WHO concluded that there was evidence from 50 studies that microscopic particles are found in water and could be absorbed by the human body.
Professor Peter Jarvis of Cranfield University, one of the report's authors, told Sky News that tap water in the UK contains between zero and 10 microplastic pieces in every litre.
But bottled water can contain "a few hundred".
"Where there is opportunity for water to interact with plastic material there is opportunity for plastic to go into the water source," he said.
"There are higher risks of exposure to plastics from bottled water than tap water. The evidence points to the cap itself as the main contributor to plastics in the water."
The WHO report says particles too small to be seen with the naked eye are likely to be absorbed by the human body but "firm conclusions" on the risk "cannot yet be determined".