Britney has a private jet on standby

A source said: "Jamie and Britney are standing by to get a call that Jamie Lynn has gone into labour and when they do, they are jumping on private jet." Although Britney - who has two children with ex-husband Kevin Federline - doesn't plan to be in the delivery room when Jamie Lynn gives birth, she has been in constant contact with her younger sister offering support and advice.

The source added: "Ever since Britney went to Jamie Lynn's baby shower the girls have been in regular contact, mostly texting, about the pending birth of Jamie Lynn's little girl." The 'Zoey 101' actress and her 19-year-old fiance Casey Aldridge are believed to be expecting a girl after she was spotted loading a pink baby carrier into her vehicle. Jamie Lynn and Casey recently moved into their new $300,000 home in Liberty, Mississippi, which is just 30 minutes away from Jamie Lynn's mother Lynne.

Some sources claim Britney flew to visit Jamie Lynn yesterday, after learning her sister had been experiencing some "health problems". Britney is believed to be prepared to stay with her mother in Kentwood, Louisiana, for up to a month, so she can be near Jamie Lynn when she brings the baby home.

This extended visit means Britney could miss crucial meetings to determine whether her father, and co-conservator, Jamie will retain legal control over her estate.