A statement from her publicist said: "The lead singer with the Irish band The Cranberries was in London for a short recording session.
"No further details are available at this time."
It added: "Family members are devastated to hear the breaking news and have requested privacy at this very difficult time."
Irish rock band Kodaline were among the first to pay tribute on social media.
In 2017 The Cranberries announced a tour including dates in Europe, the UK, and the US.
However, in May 2017, shortly into the European tour, The Cranberries had to cancel the remainder of the European dates as a result of O'Riordan's health issues.
The official Cranberries website cited these issues as being problems with the singer's back.
O'Riordan split from her husband of 20 years, Don Burton in 2014. Together with Burton, who is the former tour manager of Duran Duran, they have three children.
The singer suffered from bi-polar disorder and was spared a criminal conviction after an air rage incident in 2014.
The Cranberries shot to international fame with their 1993 debut album Everybody Else Is Doing It, So Why Can't We? and went on to sell over 40 million records worldwide.
BBC
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