Take extra care when withdrawing money as police are warning of an especially sneaky way scammers are targeting cash machines.
The alert comes from City of London Police after a series of incidents there recently, but the con could easily happen in other areas too.
People are being urged to keep their eyes peeled for unusual devices and pinhole cameras, with crimes on the up.
Four incidents involving hard-to-spot cameras have been reported in the City in just three weeks.
PC Matt Clarke, from the City of London Police crime squad, said: "The vast majority or cashpoints in London are perfectly safe - but criminals have been known to tamper with them.
"We work closely with banks, building societies, and other cash machine vendors to make sure this type of crime doesn't happen.
"Take care and stay vigilant when using cash machines in the City of London, and London as a whole.
"If you spot anything unusual about a cash machine, or if there are signs of tampering, don’t use it.
"If in doubt, try and use a machine inside a branch.
"If you spot a suspicious device when using a cashpoint, report it to the bank concerned immediately, and notify police.
"If you think a crime is in progress when you discover the device, call 999."