Video footage has emerged showing a man going for a dip in the River Ribble.
Early on Monday morning concerned Preston residents spotted the man appearing to bathe himself in the river, near the London Road bridge.
It has drawn criticism from emergency services who say entering the Ribble is dangerous.
The unknown man is seen paddling and bathing himself in the river, submerging himself completely in the muddy waters at one point.
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Neil Taylor, service delivery manager for Lancashire Fire and Rescue Service, said: “People need to be aware of the hazards presented by swimming in open water.
“There could be unknown currents or tidal concerns and of course the water temperature can be extremely dangerous regardless of the time of year. Lancashire Fire and Rescue Service attend numerous water incidents every year and unfortunately some have had tragic consequences.
“I’d like to reiterate to people considering swimming in open water to think about the potential dangers involved in swimming in open water and not take part in this type of activity.”