The risky practice puts them in danger from purse-dippers, who have already struck in Fleetwood and Cleveleys, say police.
Officers say the con artists have adopted a very plausible line of patter to disarm shoppers - and then steal their purses.
Thieves are known to have visited Fleetwood's Asda store, the Tesco stores in Cleveleys and Blackpool and Morrisons in Kirkham.
Police were alerted after one shopper reported a stolen purse.
PC Quentin Allen, who is based at Cleveleys, said: "This is a gang who we suspect are travelling the country in a car with the aim of carrying out purse-dipping missions in larger stores like supermarkets.
"The chief method is to distract victims by telling them they are foreign and struggling to understand the labels on items on a shelf, and while the person tries to help them, one of their cohorts dips into their bags and steals their purse.
"What is a worrying revelation is the number of people - and not just the elderly - who have their PIN numbers written on bits of paper in their purses.
"One person even had it written on their card.
"We cannot stress how unwise this practice is and we advise people to come up with a safer way to remember their PIN numbers."
POlice have released CCTV footage of people whom they wish to interview in connection with their inquiries.
PC Allen said: "We are urging shoppers to be vigilent because history has told us that these gangs sometimes return to areas they have visited before."
Anyone with information should phone (01253) 876611.