The "Toxic" singer is currently receiving treatment at Malibu Promises rehab centre for alleged drink problems, but doctors believe Britney is using alcohol to deal with her depression.

A source told gossip website TMZ.com: "No doubt about it - she likes to drink. But doctors think she is drinking to cope with a bigger problem."

"They strongly believe post-partum depression is the problem. Their other theory is that she is suffering from bipolar disorder."

Britney - who gave birth to son Jayden James last September, almost exactly a year after her first son Sean Preston was born - is said to be reading Brooke Shields book "Down Came the Rain."

The tome details the actress' own battle with post-partum depression.