Greenpeace says a survey of 13 UK rivers has found they all contain microplastics.
Scientists and campaigners, including Harry Potter star Bonnie Wright, sampled points along the rivers including the Exe, Thames, Great Ouse, Trent, Mersey, Aire, Derwent, Wear, Clyde and Lagan.
Of rivers which are in, or pass through, Wales the Severn, Conwy and Wye were surveyed.
Greenpeace says the River Severn was found to be contaminated with 42 pieces of microplastic collected in approximately an hour.
10 pieces were found in the River Wye and nine in the River Conwy.
The environmental campaign group says five out of 13 rivers contained microbeads - which were partially banned in 2017.
More than half the rivers tested contained plastic pellets called ‘nurdles’, which are used as a raw material in the production of plastic products.
The highest concentrations were in the River Mersey, where 875 pieces were captured in half an hour.
Greenpeace says this makes the waterway, at the time it was sampled, proportionally more polluted than the great Pacific garbage patch, considered by scientists to be one of the most plastic-polluted expanses of water on Earth.