Britney Spears is threatening to cancel her tour after receiving a string of death threats.

The 'Womanizer' singer - whose eagerly anticipated 'Circus' tour kicked off earlier this month - has become increasingly concerned the lives of her sons Sean Preston, three, and two-year-old Jayden James could be in danger following the menacing messages.

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A source told Britain's Look magazine: "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.

"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."

Britney, 27, has also been left distraught after she was criticised for not singing live during recent performances, and those close to her are concerned she may crack under the pressure.

A source said: "Britney is under so much strain, what with relentless performing, the bad reviews and the worry over her boys. It all got too much and she screamed at her team, 'I can't do this any more, why are you all torturing me?' She ran out of the room crying."