New Delhi, Mar 18 : Britney Spears is threatening to cancel her `Circus' tour after receiving death threats via the phone and email.

The ''Womanizer'' singer fears for the safety of her sons Sean Preston, 3, and Jayden James, 2, after getting menacing messages from unknown people.

"Britney received some anonymous death threats over the phone and in the mail, and her worst fear is a kidnap plot against Sean or Jayden. She''s been having terrible nightmares ever since then and she wakes up in the middle of the night crying out for her boys," a source told Britain''s Look magazine.

The source added: "She told her head of security and he thinks it''s a lone stalker. Her paranoia has reached fever pitch and she''s wracked with fear."

The 27-year-old star's eagerly anticipated ''Circus'' tour kicked off earlier this month, reports The China Daily. (ANI)