Researchers have found that young black women aged 40 or younger in the UK have a higher risk of breast cancer coming back than white women, despite having the same access to healthcare.
The reasons for the findings are unknown but could be down to biological and genetic factors, a language barrier, or a lack of awareness of breast cancer in the group all barring the way to effective treatment.
Experts called for more work to take place to find the cause.
The research published in the British Journal of Cancer also found that young black women are more likely to have larger more aggressive tumours and higher rates of triple negative breast cancer - a type of breast cancer that does not respond to hormone therapies.
Academics from the University of Southampton said that these factors did not completely explain why black women have poorer survival rates, particularly those with the type of breast cancer that is expected to be sensitive to hormones.
The study put forward a number of possible explanations why black women may be more affected than other ethnic groups.
An unidentified biological factor such as cancers in women of African-Caribbean descent responding less well to treatments, including hormone therapy, or differences in the genetics of their tumours could be a reason.
Early diagnosis may also play a role if black women are less aware of the symptoms or less likely to be breast aware - meaning the cancer is diagnosed at a later stage and reducing the chance of successful treatment.
Cultural factors such as recent immigration to the UK or language barriers could in practice affect use of health services, despite equal treatment from the NHS.
Study author Dr Ellen Copson said: "Our study confirms for the first time that black women under 41 in the UK are more likely to have breast cancer recurrence than their white counterparts, despite equal access to healthcare.
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"The finding also backs up similar findings in the USA, suggesting that this could be an international trend, but further research is needed to try and pin down the exact cause or causes, so we can tackle this issue."
Dr Julie Sharp, Cancer Research UK's head of health information, added: "It's worrying that ethnic background may be a factor influencing a woman's chance of surviving breast cancer. We know that some ethnic populations carry higher genetic risks of getting certain types of breast cancer, but if this difference is down to symptom awareness or access to healthcare, that is particularly concerning.
"More research is needed to look into the reason why young black women have higher rates of recurrence, but in the meantime women of any ethnic background should be aware of what is normal for their breasts and get any new lumps or anything unusual checked out by their GP. More often than not breast changes won't mean cancer, but it's best to get any unusual changes checked out."