Waste crime officers from the Environment Agency (EA) are appealing for information after dumped carrier bags have affected part of Sunderland's coastline
The incidents, which have affected both Roker and Whitburn beaches, have been reported to the EA over the course of a year.
The plastic carrier bags, from various supermarkets and shops, have been deliberately cut into strips and disposed of illegally.
Dave Edwardson, environmental crime team leader for the EA, said: "We are looking for information to help us track down whoever is dumping these plastic bags illegally. Sunderland Council put in a lot of hard work to keep the beaches clean but we need to find the people responsible to stop them being dumped and affecting the local coastline.
"We have been investigating potential sources including local foul and surface water sewers, waste plastics operators, the local supermarket, the port and marina for any activity but have yet to find the people responsible.
"Whoever is dumping the plastics is in breach of Environmental Permitting Regulations, the penalties for which can be an unlimited fine and up to five years imprisonment for the most serious offences at crown court."
Dave said that a flytipper might be responsible for the waste or alternatively the plastic could have been dumped from a ship.
His team also wants to hear from anyone who knows a business reason why the plastic bags would be cut up into strips.
As well as being unsightly, the cut up bags are a potential hazard to coastal wildlife. The plastic bags can also become tangled up in seaweed and remain in the area for a long time.
Anyone with information about the incidents at Sunderland should report it by either contacting the Environment Agency's 24 hour hotline 0800 80 70 60 or by calling crimestoppers on 0800 555 111.