Christina Aguilera is overjoyed after learning her chart rival Britney Spears has become a mother.

Lady Marmalade singer Aguilera admits her former The Mickey Mouse Club co-star's pregnancy hadn't really registered until she gave birth to Sean Preston on Wednesday.

She tells MTV News: "My assistant came in the room when it happened and she's like, 'Britney had a boy!' and I was like, 'Oh my gosh!'

"I don't know where I was for a while when she was pregnant but it really kicked in at that point. It's like: 'Wow, she had a baby! That's crazy!'

"I'm overjoyed and thrilled for her. It's such an exciting time in her life and I just wish her the best."

Aguilera says she has already sent Spears a baby gift and a card.