Johnson, who helped lead the Leave campaign in the 2016 Brexit referendum, is caught on camera starting to recite Kipling's poem, Mandalay, after striking a bell at the Shwedagon pagoda in Yangon.
"The temple bells they say," Johnson says in television footage by Channel 4. "Come you back, you British soldier."
As Johnson continues with his recitation of the poem which celebrates a soldier's love affair with a local woman during Britain's colonial rule of what was then known as Burma, the ambassador tenses.
"You're on mic. Probably not a good idea," British ambassador Andrew Patrick said.
"What, The Road to Mandalay?" Johnson asked.
"No. Not appropriate," the ambassador said.
A spokeswoman for the Foreign Office declined to comment.
Prime Minister Theresa May's appointment of Johnson, who in the run-up to Britain's referendum on EU membership compared the goals of the European Union to those of Adolf Hitler and Napoleon, caused consternation in European capitals.
In recent weeks, Johnson has attempted to set out his vision of Britain outside the European Union.
(Reporting by Guy Faulconbridge; Editing by Stephen Powell)
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