Invisible microplastics have been found almost 400 metres underground in UK water streams, according to the results of a citizen science project conducted by wild swimmers.

More than 100 outdoor swimmers in the UK became “waterloggers”, collecting water samples from their favourite place for a dip using empty glass wine bottles.

This water was then tested, with microplastics present in every single sample.

One of these samples was taken 400 metres underground in a cave in Northamptonshire. Rebecca Price, a caver who collected the samples deep underground, said, “The cave sample was taken from an underground waterfall which filters through natural rock. I’m shocked to find that nano- and microfibres were found that deep underground.”

She also collected the samples with the highest number of microplastics, at 155 pieces a litre, in the River Nene, Northamptonshire, where she swims frequently.

She added: “The Nene has had very bad reports about its water quality in recent years. These results focus on microplastics and highlight another toxic silent contaminant choking our beautiful river.”

Laura Owen Sanderson, the founder of the non-profit We Swim Wild, which carried out the sampling, said: “We now know that microplastics are infiltrating every aspect of our lives. We breathe in, drink and eat plastic particles every day; and little research has been done to establish what risk that poses to human health.

“This campaign provides a large and unique grassroots dataset for the UK government, as clear evidence that urgent action is needed now.”

The group is calling for the government to test regularly for microplastics in UK rivers, and will soon launch another 12-month study into invisible contaminants in waterways.

Recent research by Outdoor Swimmer Magazine found that wild swimmers are hugely concerned by pollution, and more than one-third of swimmers surveyed had written to their MPs and supported campaigns over the problem.

Michelle Walker, the technical director at the Rivers Trust, told the magazine: “What really stands out to me is how swimming outdoors motivates people to take direct action on water pollution, and we’ve really seen the impact of that in the last year. Tens of thousands of people contacted their MPs to demand amendments to the environment bill, and as a result government were forced to change direction.”