Editorial teams were "up all night" checking through entries
It was once billed the "Patriarch of the Library" but Encyclopaedia Britannica has proven to have a less than orthodox view of the Irish Civil War.
A concise version of the reference work first published seven years ago says the 1922 conflict was between Catholics in the south and Protestants in the north.
As any Irish schoolchild, or indeed anyone who has seen the film Michael Collins knows, it was in fact a conflict fought by those in favour of the 1921 Anglo-Irish treaty and those opposed.
The war was fought among Catholic nationalists in the south. Northern Protestants had no involvement.
The glaring blunder was carried on a hand-held device first sold six or seven years ago but was only spotted this week.
Ian Grant, managing editor of Encyclopaedia Britannica, said the mistake was "rare" and may have been caused by an editor attempting to condense complex history.
He added that they may have confused the Irish Civil War with the troubles in Northern Ireland.
Mr Grant added that editorial teams in London and Chicago were confident that online databases, which are updated regularly, do not carry other mistakes on Irish history.
He said they had been "up all night" checking through entries.
He added: "It's important to get this thing right. If there is a mistake Britannica will always admit it and correct it."
Encyclopaedia Britannica contains about 64 million words of text from 4,500 contributors around the world and is put together by about 100 editors.
Their offices can receive up to a dozen complaints or queries each week mainly concerning dates.
The Department of Education in the Irish Republic announced last month that the country's 4,000 schools would have free access to Encyclopaedia Britannica online databases as part of an e-learning initiative.