The River Mersey is more polluted than the great Pacific garbage patch, according to research from Greenpeace.
The environmental group said a survey of 13 UK rivers found they all contain microplastics, with the River Mersey in particularly poor condition.
Greenpeace spokeswoman Fiona Nicholls told talkRADIO’s Matthew Wright the study made for “miserable reading”.
“In the Mersey we found two million microplastics per square kilometre,” she said.
“That compares to the great Pacific garbage patch which is widely considered the most polluted expanse of water on Earth.”
The research found UK rivers including the Thames, Severn and Trent all had tiny pieces of plastic measuring less than 5mm.
The highest concentration was in the River Mersey, where 875 pieces of plastic were collected in just half an hour.
Ms Nicholls said it is “relatively impossible” to remove such plastics once they enter waterways.
“It could start off as a bottle or as a piece of packaging from supermarket, and once it’s made its way into the river, or indeed out to sea the natural environment bashes them around and they become smaller and smaller and then they become microplastics,” she said.
“Microplastics are relatively impossible to remove from the environment once they’re in there.”
She added: "With every minute more plastic is getting pumped out into our rivers and eventually out to sea."
Greenpeace has called for the government to introduce “bold” plastic reduction targets and create an independent watchdog with powers to enforce them.