Susan Boyle has been given extra security after a middle-aged woman began stalking her, it's been reported.

The American woman is understood to have sent Boyle, 48, a series of letters before unexpectedly turning up at her house in Scotland.

The incident is said to have left the Britain's Got Talent star “shaken up” and forced Simon Cowell to increase he security.

"Simon is not taking any chances,” a source told The Sun newspaper.

"This woman is constantly following Susan around the world. It is a middle-aged American woman who has become obsessed with her.”

Boyle is currently in the US promoting her debut album, I Dreamed A Dream, which is released later this month.

On Tuesday, the singer wowed fans with a performance on Dancing With The Stars.

Susan Boyle - Through the Years