Best selling author and historian Tim Pat Coogan has been forced to cancel a book tour in the U.S. for his new book 'The Famine Plot' about how the British government mishandled the Famine.

The Coogan refusal comes as a major shock as he is considered the foremost authority on figures such as Michael Collins and Eamon de Valera.

He is one of Ireland's best known historical writers, authoring such books as Ireland Since the Rising (1966), The IRA (1970), On the Blanket (1980), Wherever Green is Worn (2000), Ireland in the Twentieth Century (2003).

He has written widely acclaimed biographies of both De Valera and Collins and is one of the best known Irish figures in America. He is a former editor of the Irish Press newspaper.

Coogan was due for a North East tour of the U.S. to promote his new book published by Palgrave MacMillan.

He was set to deliver a lecture at the American Irish Historical Society in New York but they were informed on Tuesday that he was not given a visa.

It is one of a number of recent visa decisions at the Dublin Embassy which have been causing concerns at senior levels of the Irish government.

Experts say it appears that seemingly arbitrary rules are being followed with no definitive explanations.

How a historian of the stature of Coogan is refused access to the U.S. where he is a very frequent visitor and a well known name is a major mystery to most observers and will be seen by some as a major blot on the American Embassy and Ambassador Dan Rooney.