David Dimbleby issued a stern telling off as they giggled when he made clear it was not a laughable matter.
It happened as the BBC host took on the Labour Party's Diane Abbott over the conflict in Syria after the regime used chemical weapons against its own people.
Dimbleby said: “We’ve had two American Presidents, Obama in 2012, saying the ‘red line’ was chemical weapons, nothing happened.
“We now have President Trump that this chemical attack crosses many many red lines, many many red lines repeated so you’ve got…”
David Dimbleby put down the audience for laughing at his Trump impression
And as the audience burst out laughing at his President Trump impression, he snapped: “It’s not funny, it’s the way he talks!”
The moment came just hours before the Republican President last night ordered a US missile strike on an air base in Syria.
In a speech to the American people, he said preventing and deterring the use of chemical weapons was in the "vital national security interest of the US".
President Trump said: "On Tuesday, Syrian dictator President Bashar al-Assad launched a horrible chemical attack on innocent civilians.
"Using a deadly nerve agent, Assad choked out the lives of helpless men, women, and children.
Horrific images depict sickening chemical gas attack in Syria Tue, April 4, 2017
At least 58 people, including nine children, were killed in an air raid that released "toxic gas" on the rebel-held Syrian town of Khan Sheikhoun
EPA 1 of 13
A child receiving treatment at a field hospital after an alleged chemical attack in Idlib, northen Syria
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It’s not funny, it’s the way he talks!
“It was a slow and brutal death for so many. Even beautiful babies were cruelly murdered in this very barbaric attack. No child of God should ever suffer such horror.
“Tonight I ordered a targeted military strike on the airfield in Syria from where the chemical attack was launched.”
Sir Michael Fallon on Friday said the US had the “full support” of the UK Government after launching the airstrike as he said the action was “appropriate”.
Speaking to BBC Radio 4’s Today Programme, he said: “Our reaction is we fully support what the Americans have done.
“It’s limited, it’s wholly appropriate, it targeted the airfield… the equipment the Americans believe was used in this gas attack and is designed to deter the regime from planning similar gas attacks.”
During the programme, the BBC host insisted a “red line” had been crossed by launching a missile strike in Syria and the US had “effectively declared war on the Assad regime”.
This prompted a stark response from the Cabinet Minister as he insisted the missile strike was intended to deter any future use of chemical weapons.
Sir Michael said: “No, it hasn’t declared war. The Americans have made it very clear that the attack last night was limited, it was narrowly focused.
Trump launches military strike against Syria Fri, April 7, 2017
U.S. President Donald Trump launched 59 Tomahawk missiles against Syria in response to its chemical attack on its own civilians
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Trump ordered a massive military strike against a Syria
“They did everything possible to minimise civilian casualties indeed to avoid Russian personnel on that particular airfield.
“This is the first gas attack that President Trump has been faced within the administration and he has chosen to respond in this particular way. To take action to deter the regime from carrying out similar attacks in the future.”
However speaking to LBC, Sir Michael said the possibility of the US launching another airstrike could not be ruled out.
“No, you can’t rule out further airstrikes,” Sir Michael confirmed. “But the President made clear in his appeal last night he now wants everybody to get involved, to redouble their efforts, to bring about a political settlement without Assad in Syria to finally bring that civil war to an end and to have that country living in peace.”