Boris Johnson heckled David Cameron as he directly attacked the London Mayor's position during his speech on the upcoming EU referendum.
During the speech, the Prime Minister described the referendum as a "straight democratic decision" to leave or stay, with no second negotiation followed by a second referendum.
Mr Johnson had suggested a no vote might not necessarily result in the UK pulling out of the EU altogether, and instead could create a "new relationship based upon trade and cooperation".
As Mr Cameron made his attack on the Mr Johnson's position, he reportedly shouted "rubbish, rubbish".
The Prime Minister ended his speech with a second dig at Mr Johnson, saying: "I'm not standing for re-election. I have no other agenda than what is best for our country."
Boris Johnson announced his decision to go against David Cameron and "advocate Vote Leave" on Monday.
He described the EU as "a political project that has been going on for decades, and is now in real danger of getting out of proper democratic control".
