It is believed the letters could be coming from angry animal rights
campaigners who are furious with Madonna for wearing a £35,000 coat made out
of chinchilla fur to a recent glitzy party.
The pop superstar - who splits her time between her homes in the US and the
UK - has now beefed up her security on the advice of her management team.
A source said: "Madonna has been receiving some really disturbing and
threatening mail. Obviously, she feels very scared and has had lengthy
conversations with the police about it. Madonna's people have been growing
increasingly concerned about her safety and have been left with no option
but to step up her security."