London, July 18 - Pop singer Britney Spears is refusing to take her sons on tour to Russia after she received a series of death threats by e-mail.

The 'Toxic' hit-maker will base herself in Finland and fly in just for the two concerts in St. Petersburg and Moscow next week, reported thesun.co.uk.

'Originally, both boys were to accompany Britney but she was against taking them because of these constant threats,' said a source.

After the Moscow performance, Spears will fly straight to London in her private jet to be reunited with sons Sean Preston, three, and Jayden James, two.

The identity of the e-mail sender is unknown.