Lancashire County Council Trading Standards is warning people to beware of high-pressure salesmen trying to overcharge them for bags full of fresh fish which they don't really want.
Every year officers receive complaints about traders who mainly target elderly people, conning them into paying sometimes hundreds of pounds for large quantities of fish.
The salesmen are usually from the North East and are known to move around the region selling the fish from the back of an unrefrigerated van.
The alert for Lancashire residents comes following reports that they have been operating in neighbouring areas earlier this week.
The traders often try to convince people to buy just a few pieces of fish, but then present them with freezer-filling quantities, or use a card machine to charge people's cards for large amounts without their agreement.
County Coun. Albert Atkinson, deputy leader of Lancashire County Council, said: "These traders operate by moving around the north, quickly going from area to area to try to avoid detection.
"We've been alerted that they've been operating nearby this week, with people being charged over £250 for a couple of bags of fish, so are warning people to avoid doing business with them and keep an eye out for their neighbours, friends and relatives.
"The sellers are usually trying to quickly get rid of an overabundance of fresh fish, but use a variety of unscrupulous methods to take advantage of people.
"They usually target estates where many of the residents are elderly and vulnerable. Our advice is never to deal with cold-callers on your doorstep, whatever they happen to be selling."
Anyone with concerns about doorstep callers in their area should call Consumer Direct on 03454 04 05 06.