Nicolas Sarkozy called Israeli Prime Minister Benjamin Netanyahu a "liar" in a private conversation with Barack Obama, according to an exchange overheard at the G20 summit in Cannes.
A sympathetic Obama replied to Sarkozy: "You're fed up with him? I have to deal with him every day."
The two leaders had failed to realise their microphones were still on when they retired to a private room after a press conference. A Reuters journalist present at the summit confirmed that the reports, first published on a French website Arrêt Sur Images, were true.
Their censorious conversation lasted for three minutes before the leaders' realised their mistake, according to French journalists.
Apparently the comments were triggered by Obama expressing disappointment that the French had supported Palestinian membership of the UN cultural and education agency Unesco.
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It's not the first blunder for forgetful politicians and their microphones. Gordon Brown famously grumbled "that bigoted woman" about a voter he'd just met.
George W Bush had a special talent for summation, reportedly greeting the former prime minister as "Yo Blair" and condensing international relations to "we just need to get Syria to get Hezbollah to stop doing this s**t".
US President Ronald Reagan caused alarm during the height of the Cold War after something he said to 'test' the microphone was broadcast live. Take a look at five of the best microphone gaffes.