Fewer women survive breast cancer in the UK than in Australia, Canada, Norway and Sweden, research has shown.
Three year cancer survival is 87%-89% in the UK compared with 91% to 94% in the other four countries, according to new figures.
Denmark also lags behind with similar survival rates to the UK. But evidence suggests that, unlike in Britain, this is due to women being diagnosed late.
Researchers analysed data from 257,362 women diagnosed with breast cancer between 2000 and 2007 and recorded in cancer registries. The findings are published in the British Journal of Cancer.
Women were just as likely to be diagnosed at an early stage in the UK as in the other countries, said the scientists. However, the chances of not dying from breast cancer were lower for women with late-stage disease.
One year after diagnosis, survival for women with early-stage disease was close to 100% in all countries. Older women with breast cancer and women with more advanced disease may be treated less aggressively in the UK, said the researchers.
Lead researcher Dr Sarah Walters, from the London School of Hygiene and Tropical Medicine, said: "The reasons for low overall survival in the UK and Denmark are different.
"In Denmark, women are diagnosed with more advanced disease, but survival at each stage is similar to that in other countries. In the UK, women are diagnosed at a similar stage as elsewhere, but survival is lower than women with the same stage of disease in other countries."
"In the UK, we should now investigate whether the treatment of women with later-stage breast cancer meets international standards. There is particular concern that this is not the case, especially for older women."
Sara Hiom, Cancer Research UK's director of early diagnosis, said: "International comparisons like this are vital in helping us better understand what is influencing cancer survival. We're beginning to see some important clues now, but while we're closing the survival gap for breast cancer, UK women continue to fare worse than in these other countries."