Adding a cheap and safe drug to standard chemotherapy could help half of women with breast cancer live longer, scientists suggest.

In a new study, published in the journal Nature, British and Australian researchers say preliminary findings indicate the hormone progesterone could be used to slow the growth of some tumors, BBC News reports.

They called the findings are "very significant" and they are planning clinical trials.

Hormones are known to play a huge role in breast cancer, promoting tumor growth.

But the researchers with the University of Cambridge and the University of Adelaide found cancer cells growing in the laboratory grew to half the size when treated with progesterone and the standard breast cancer drug tamoxifen than when given tamoxifen alone.

"It appears you control the tumors better, but to prove it is better in women with breast cancer we need to do the trial,” said researcher Carlos Caldas from the University of Cambridge. "It could be very significant. In early breast cancer you could increase the number of people being cured and in advanced breast cancer, where we're not curing, we could control the disease for longer."