Following Downing Street's threats to close loopholes used by the rich to avoid tax, charities fear they may lose their largest donors.
Prime Minister David Cameron said that multi-millionaires were using donations to charities that do little work to wipe out their income tax bills.
Chancellor George Osborne has been "shocked" by the level of tax avoidance and plans to limit tax breaks.
But a spokesman for a big charity told us this morning: "It's a well-known fact that most of our donors are fat buggers - and I mean really large! Not petite... you know, the kind of people who wear XXXXl trousers and stuff.
"If we lose their donations there will only be the thin people left and there's not nearly as many skinny millionaires as fat ones.
"This is a very worrying development."