Preston Police inspected the cash machine at the Tesco Express store on Church Street in Preston on 2 February.
They advised a card reading machine had been attached to it. It is possible unauthorised transactions may have been made on peoples bank accounts.
Police are asking anyone who used the machine in the last week to check statements and contact their bank. Similar messages went out to local businesses and office workers in the immediate vicinity on Monday morning.
Police initially attached a warning notice to the device but this was removed when the cash machine was declared safe to use again earlier this afternoon.
Police remind everyone it is important to be vigilant when using cash machines and be aware of who is near by.
A card reading skimmer is a portable capture device that is attached onto the legitimate scanner. The skimmer passively records the card data as you insert your card into the real scanner. Card thieves will often temporarily attach the card reader device to ATMs as they are easy to retrieve later.