Boris Johnson described himself as “a bit of a Europhile” and his Euroscepticism as “a bit of fun” in private comments to Ireland’s former EU commissioner Peter Sutherland in 2013.

The then mayor of London made the remarks to Mr Sutherland at a private dinner after the former Irish attorney general chaired a debate at the London School of Economics on the UK’s future on December 10th, 2013, less than three years before Mr Johnson campaigned for a Leave vote in the Brexit referendum.

Details of the private conversation between the two men is contained in a new book, The Globalist - Peter Sutherland, His Life and Legacy written by journalist John Walsh that will be published later this week.

Mr Walsh writes that, reacting to Mr Johnson’s comment, Mr Sutherland, a fervent supporter of the European Union, “found it hard to believe that a campaign of crucial importance for both the UK and the EU would be conducted in such a glib manner by some of the leading figures in the Leave camp.”

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The book reveals that Mr Sutherland’s role as a UN special representative for international migration made him “a useful bogeyman” for the Leave campaign that was “about taking back control.”

Louise Mensch, a former Tory MP and prominent Brexiteer, told one of Mr Sutherland’s associates that the Leave campaign was “delighted” the Irish businessman, who lived in London, had become one of the public supporters for the Remain side in the referendum campaign because of his close association with migration.

Mr Sutherland, then director general of the World Trade Organisation, was considered as a potential successor to Jacques Delors as European Commission president in 1995 but the French government opposed him and the Dubliner, a Fine Gael man, did not receive the backing of the then Fianna Fáil-Labour coalition.

The book discloses that in 2004 Mr Sutherland turned down a full honorary knighthood because he was “not very comfortable with it” and he was advised by then secretary general of the European Commission, Irishman David O’Sullivan that it would “kill you in Irish terms” if he ever wanted to represent his country.

Mr Sutherland would have been entitled to accept a full knighthood, offered for his services to philanthropy, because he was born in Ireland in 1946, three years before it became a republic.

In one colourful exchange recounted in the book, Mr Sutherland pinned the Japanese ambassador against a wall during eleventh-hour brinkmanship before the signing of the Uruguay round of trade talks in 1993.

Shane Sutherland, his son, recalls his father having toy guns purchased by Mr Sutherland for his children confiscated at Dublin Airport and later having them released because he was attorney general at the time.