Microplastics in UK rivers are in higher concentrations downstream from wastewater treatment plants, according to new research.
The tiny pieces of plastic have become one of the major focuses of the current worldwide concern about the pollution caused by single-use plastics.
Scientists from the University of Leeds have undertaken what they say is one of the first studies to determine potential sources of microplastics pollution.
The team measured concentrations up and downstream of six wastewater treatment plants in the north of England and found an increase in all cases.
And the researchers also found that 90% of the microplastics they found were in the form of fragments and fibres rather than microbeads, leading them to warn about the role of synthetic fabrics in long-term environmental harm.
The team found that, on average, the concentration of microfibres was up to three times higher downstream of treatment plans but, in one instance, it was up by a factor of 69.
Paul Kay, from the School of Geography at Leeds, said: "By categorising the types of microplastics we can identify what aspects of our lifestyle are contributing to river pollution.
"Not that long ago microbeads in toiletries and cosmetics were the microplastics getting all the public attention.
"Seeing the amount of plastic microfibres from clothing and textiles polluting our rivers, we need to think seriously about the role of our synthetic fabrics in long-term environmental harm."
Microplastics come from a wide range of materials including tiny plastic beads found in health and beauty products, plastic fibres from clothing and plastic flakes that break down from packaging.
They have been found in some of the most remote environments on Earth.
Last week, a study showed how microplastics or other debris were found in 100% of wild mussels sampled from eight coastal locations around the UK and those purchased from eight unnamed supermarkets.
Dr Kay, whose study is published in Environmental Science and Pollution Research, said microplastics are one of the least studied groups of contaminants in river systems.
He said: "These tiny plastic fragments and flakes may prove to be one of the biggest challenges in repairing the widespread environmental harm plastics have caused.
"Finding key entry points of microplastics, such as wastewater treatment plants, can provide focus points to combating their distribution."
But Dr Kay said "pervasive microplastics" were still found in the upstream samples.
He said: "So, while strengthening environmental procedures at treatment plants could be a big step in halting their spread, we cannot ignore the other ways microplastics are getting into our rivers."
Dr Kay said that as well as providing an entry point for microplastics found in wastewater - such as clothing and textile microfibers that shed into washing machines - treatment plants may also contribute secondary microplastics as a result of plastics breaking down further during their processes.