People living in Lincolnshire have been warned to be on their guard against phone calls from conmen posing as charity workers.
The Mothers' Union in Boston said they had received reports of high-pressure calls claiming to be from the Christian Aid charity asking for at least £50.
They said that when people refused, the callers asked how their target could "live with themselves".
Christian Aid workers said they would never collect money in this way.
A spokeswoman for the Mothers' Union said: "When refused, the callers immediately started talking about conscience and how could the person live with themselves.
"But it sounded wrong. The language they were using, the pressure they were using and the amount they were asking for - more than £50 - sent alarm bells ringing."
The Reverend Brian Newton, the co-ordinator for Christian Aid in the Boston area, said: "We do not make these sorts of phone calls. There is something dirty about this, it is criminal, it is appalling."
Lincolnshire Police have been informed.