Madonna has been forced to postpone her 50th birthday celebrations after twisting her ankle.

The '4 Minutes' singer - who reaches the milestone age on Saturday (08/16/08) - was set to throw a lavish party at her country estate in Wiltshire, England, but has now postponed the festivities after suffering an injury during rehearsals for her forthcoming world tour.

A source told Britain's Daily Mirror newspaper: "Plans are very hush-hush but Madonna is definitely going to celebrate in style.

"Unfortunately, not everything has gone as expected as she's had many other things on her mind - not least rehearsals for her 'Sticky and Sweet' tour. On top of all the stress, she twisted her ankle over the weekend.

"She views this as a sign she should postpone the birthday bash and play it by ear instead.

"Madonna now thinks it will go ahead during a three-day break in the tour towards the end of the month."

Among the guests invited to the party - which the singer's husband, director Guy Ritchie has helped plan - were officials involved with the couple's adoption of Malawian David Banda.

The source added: "Guy wants the party to be traditional and British. He has already ordered crate loads of real ale. He's also arranged for a folk band to play and the people invited - Gwyneth Paltrow, Stella McCartney and Trudie Styler - are basically friends Madonna's known for years.

"Everyone else will be immediate friends and family, plus a few people from Malawi that Madonna will be flying out. A few of her Kabbalah associates are also expected and Madonna will make sure only Kabbalah water will be served."