Tests on major brands of bottled water have found that nearly all of them contained tiny particles of plastic.
In the largest investigation of its kind, 250 bottles bought in nine different countries were examined.
Research led by journalism organisation Orb Media discovered an average of 10 plastic particles per litre, each larger than the width of a human hair.
Companies whose brands were tested told the BBC that their bottling plants were operated to the highest standards.
The tests were conducted at the State University of New York in Fredonia.
Sherri Mason, a professor of chemistry at the university, conducted the analysis and told BBC News: «We found [plastic] in bottle after bottle and brand after brand.
«It’s not about pointing fingers at particular brands; it’s really showing that this is everywhere, that plastic has become such a pervasive material in our society, and it’s pervading water — all of these products that we consume at a very basic level.»
Currently, there is no evidence that ingesting very small pieces of plastic (microplastics) can cause harm, but understanding the potential implications is an active area of science.
Commenting on the results, Prof Mason said: «It’s not catastrophic, the numbers that we’re seeing, but it is concerning.»
Experts have told the BBC that people in developing countries where tap water may be polluted should continue to drink water from plastic bottles.
Contacted to comment on the findings, the companies behind the brands have insisted that their products meet the highest standards for safety and quality.
They also point to the absence of any regulations on microplastics and of the lack of standardised methods of testing for them.
Last year, Prof Mason found plastic particles in samples of tap water and other researchers have spotted them in seafood, beer, sea salt and even the air.
This latest work comes amid growing international attention on plastic, fuelled by the BBC’s acclaimed Blue Planet 2 series in which Sir David Attenborough highlighted the threat of plastic waste in our oceans.