In light of the London terror attack last weekend that killed seven people, the mayor posted on his Twitter account that there was "no reason to be alarmed".
However, President Trump snapped back: "At least seven dead and 48 wounded in terror attack and mayor of London says there is 'no reason to be alarmed!'"
After Khan replied that he had "more important things to do than respond to Donald Trump's ill-informed tweet that deliberately takes out of context his remarks".
However, Trump didn't leave it there, accusing him of making a "pathetic excuse".
President Trump accused Khan of making a "pathetic excuse"
Khan joked that he is "too busy" to reply to Trump's tweets
I'm too busy to reply to his tweets, isn't he busy?
This morning Khan was forced by hosts Piers Morgan and Susanna Reid to finally address the online spat once and for all.
The former MP for Tooting argued: "We're not kids in the playground, he's the president.
"I'm too busy to reply to his tweets, isn't he busy," Khan joked.
A fired-up Morgan told Khan: "I don't think it helps anyone [to have these spats], because actually there's a bigger picture."
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Sadiq Khan leads another ANTI-TRUMP parade in London Mon, March 6, 2017 PA 1 of 5
Annie Lennox, Mayor of London Sadiq Khan and Bianca Jagger lead crowds across Tower Bridge during the March4Women event in London, ahead of International Women's Day
The mayor cooly replied: "It takes two to tango and I'm not tangoing with this guy. I've got bigger things to do."
This came after Morgan and Reid grilled Boris Johnson over returning ISIS fighters.
The Foreign Secretary became unstuck after Reid accused the government of only using the power to stop potential fighters returning to the UK.