The President of Uruguay has been caught on microphone calling his Argentine counterpart, Cristina Fernandez, an "old hag".
Jose Mujica was caught at the beginning of a news conference while speaking quietly with another official during a visit to a farming area in Uruguay to discuss trade with Brazil and Argentina.
The microphone picked him up saying: "This old hag is worse than the cross-eyed man."
The "cross-eyed man" is apparently a reference to Mrs Fernandez's late husband, the former president of Argentina Nestor Kirchner, who had a lazy eye. He died suddenly of a heart attack in 2010.
El Observador newspaper, which posted the audio on its website, said Mr Mujica was referencing the Kirchners and did not realise that the microphones were on.
The clip attracted so much web traffic it shut the newspaper's website down.
Argentina's government has responded with an official protest, calling the comments "unacceptable."
The Argentine Foreign Minister, Hector Timerman, said the remarks were "denigrating" and added that they offended the memory of the dead.
The Uruguayan ambassador Guillermo Pomi was summoned and presented with a protest letter, which said: "The Foreign Ministry would like to let the Uruguayan Embassy know of the deep discomfort created by President Jose Mujica's statements regarding late President Nestor Kirchner."
However, Mr Mujica told the online version of La Republica newspaper that he had not talked about Argentina and was not going to clarify anything.
Mr Mujica, a popular president, was elected in 2009. He shunned his grace and favour house and instead lives on a farm on the outskirts of Montevideo with his wife and three-legged dog.
Known as "the poorest president in the world", the former left-wing revolutionary gives 90% of his income to charity.
Mrs Fernandez, whose frequent rhetoric about taking back the Falklands Islands continues to provoke David Cameron, has refused to comment on the incident.
Mr Mujica is not the only leader to fall prey to an open-microphone gaffe. George W Bush memorably greeted Tony Blair with "Yo Blair" at a G8 summit in 2006, while Gordon Brown was caught calling a 66-year-old widow a "bigoted woman" on the 2010 election campaign.