Robert De Niro has criticised Donald Trump again, calling the US president a “total loser” and “wannabe gangster”.

The actor has been involved in a long-running dispute with Trump, saying “fuck Trump” at the Tony awards last year.

Trump responded by saying De Niro was “a very low IQ individual”.

During an appearance on the Late Show With Stephen Colbert on Friday, De Niro told the host he had tried giving Trump a chance. “That’s what I said right after he was elected. Give him a chance. I give everybody the benefit of the doubt. This guy has proven himself to be a total loser.”

De Niro is one of the most revered actors of his generation, winning two Oscars for his performances in The Godfather Part II and Raging Bull. He will return to the gangster genre for The Irishman on Netflix, which will reunite him with the director Martin Scorsese.

When Colbert asked De Niro why audiences were so enamoured with the gangster genre, he replied: “People like the outlaw type thing, except we have a wannabe gangster in the White House now.”

De Niro also called Trump a dumbbell, saying: “Even gangsters have morals and they have ethics. They have a code and you know when you give somebody your word it’s your word because it’s all you have is your word. This guy, he doesn’t even know what that means.”

In an interview with the Guardian in January De Niro was equally scathing about Trump, calling him a white supremacist. “I thought maybe as a New Yorker he understands the diversity in the city but he’s as bad as I thought he was before – and much worse. It’s a shame. It’s a bad thing in this country.”