Fans of pop star Britney Spears are worried about her health again after she collapsed during a rehearsal on the European leg of her Circus world tour.

The hitmaker was practising a dance routine for a new song when she keeled over, suffering a bruise to her chest.

"She collapsed when the dancers went to put her back on her feet. She fell to the floor and landed on one of the dancers' legs," said a source.

"The dancer's tennis shoe smacked Britney in the chest and on the face. It hit her chest with such a force that it caused an 11 centimetre bruise," the source said.

Britney's hectic tour schedule and low blood-sugar levels were said to be responsible for the incident.