John Stewart has come forward to defend Stephen Colbert for his recent abusive on-air tirade directed at US President Donald Trump. The former Daily Show host made an appearance on The Late Show on 9 May during which he teased his former protégé for his "pottymouth" before explaining why comedians should be allowed to insult.

"I don't feel comfortable," Stewart said while a guest on the CBS show. "I know you're comfortable, I don't feel comfortable. I'll tell you why I don't feel comfortable – I've been reading about you. I've been seeing you in the news... You have a potty mouth."

The retired Comedy Central talk show host was referring to Colbert's monologue on 1 May when he called Trump "the glutton with the button", "presi-dunce", and most harshly described him as "Vladimir Putin's c**k holster".

The remarks gave rise to a #FireColbert campaign online and triggered an investigation by the US Federal Communications Commission (FCC), an independent agency that regulates interstate communications via radio, TV, cable, etc.

While accepting that he could have refrained from using crude language, Colbert defended his statements. "He, I believe, can take care of himself. I have jokes; he has the launch codes. So, it's a fair fight," he said on a recent episode.

During his guest appearance on 9 May, Stewart also came to his friend's defence. "The things that you say, even if they're crass, even if they in some ways are not respectful enough to the office of the presidency – we can insult. He can injure," said Stewart, referring to Trump.

"It's the difference between insult and injury. And for the life of me, I do not understand why in this country, we try and hold comedians to a standard we do not hold leaders to. It's bizarre."

Other former Daily Show correspondents Samantha Bee, John Oliver, Ed Helms, and Rob Corddry also joined in to celebrate Colbert's 20 years since starting out at the Stewart-led show.

Watch John Stewart's appearance on The Late Show below: