David Cameron was given a slap on the wrist by the speaker, John Bercow, after losing his cool during prime minister's questions and calling shadow chancellor Ed Balls a "muttering idiot".
It is well known that there is no love lost between Balls and Cameron, who once described the former as "the most annoying person in modern politics", but this is the first time that Cameron has let his irritation bubble over in the house.
Balls, who takes great pleasure in heckling Cameron whenever he speaks in the Commons, clearly got Cameron's back up while he was discussing the country's economic future.
Cameron said: "What we need to do is combine the fiscal deficit reduction, which has given us the low interest rates, with an active monetary policy, structural reforms to make us competitive and with inventive ways to use our hard-won credibility - which we wouldn't have had if we had listened to the muttering idiot opposite me."
The insult was met with an uproar from both benches. The Conservatives laughed while Labour members shouted the nickname "Flashman", a posh bully alter-ego they use to mock Cameron.
Bercow waited for the room to settle before asking Cameron to "please withdraw the word idiot", as it was "unparliamentary".
Cameron replied: "I will replace it with "the man who left us with this enormous deficit and financial crisis".
Balls tweeted after the outburst: "For the record, I was simply asking the prime minister, as he boasted the economy was back on track, 'tell us about the recession'."
Balls is alleged to have repeatedly called for Cameron to "chillax" and "have another glass of wine", in reference to a recent biography that described his relaxation techniques, such as playing the game Fruit Ninja.
The flashpoint followed a heated exchange between Cameron and Labour leader Ed Miliband, who accused him of bringing back the "nasty Tory party", in reference to possible alterations to employment law.