People are being warned to be vigilant after reports of bogus charity collectors across the Five Towns.
Yorkshire Air Ambulance (YAA) said it had received several calls from residents who asked if the charity had sent people out door knocking.
The bogus callers were asking people to sign up to a lottery scheme.
But the charity said that it does not use door knocking as part of its fundraising techniques.
A spokesperson said: “The YAA would never send any staff or volunteers door knocking.
“This is something we have never done, nor do we have any imminent plans to do so.”
For more information or to donate to Yorkshire Air Ambulance visit www.yorkshireairambulance.org.uk