Comedian Seth Rogen really doesn't like his fellow Canadian Justin Bieber.

In January, the Knocked Up actor went on the offensive in a tweet following Bieber's arrest in Florida for driving under the influence.

Rogen, talking on shock jock Howard Stern's radio show, said it all began backstage on a German talk show when Bieber's people asked him if he would meet him.

"I was like, 'yeah, sure, I'll meet him'. So I went outside to meet him, and he was acting like I'd asked to meet him!" said Rogen.

Rogen also described Bieber as "a good example of someone who you meet who you think you are going to hate, and then you get to hate him".

Zac Efron – who stars with Rogen in new comedy Bad Neighbours – was a similar kind of teen heartthrob, but meeting him was an entirely different experience, according to Rogen.

"When I met Zac Efron, one of the most endearing things is he recognised that as a Disney star, I as a man in my late 20s, early 30s, would f**king hate him," he said.

"Because why wouldn't I? Justin Bieber had zero f**king percent of that... no humility, no awareness."

In March, Rogen explained again why he doesn't like the star, telling chat show host Andy Cohen: "He seems like he's obnoxious and ungrateful, insincere, and he puts people's lives in danger with his horrible, horrible behaviour, and overall he acts like a piece of s**t."