US news outlet CNN has refused to air President Donald Trump’s new advertisement slamming mainstream media, calling the ad “false”.

The new television ad, entitled “First 100 Days”, highlights Mr Tump’s first 100 days in the Oval Office and list of accomplishments, while branding mainstream media as “fake news”.

CNN declined to run the clip, saying it would only accept it for broadcast if the “fake news” segment was removed.

“CNN requested that the advertiser remove the false graphic that says the mainstream media is ‘fake news’,” CNN said in a statement released on Tuesday.

“The mainstream media is not fake news, and therefore the ad is false.

“Per our policy, it will be accepted only if that graphic is deleted. Those are the facts.”

However, the Trump administration took offence at CNN’s decision, accusing the cable broadcaster of censorship.

“The campaign is continuing President Trump’s approach of reaching out to the American people by directly highlighting his work over the first 100 days with this ad as well as fighting back against the continued media bias,” the Trump campaign said in a statement.

Watch the ‘100 days of Trump’ advert below:

Trump executive director Michael Glassner said: “It is absolutely shameful to see the media blocking the positive message that President Trump is trying to share with the country.

“It’s clear that CNN is trying to silence our voice and censor our free speech because it doesn’t fit their narrative.”

Why is ‘fake news’ key to Donald Trump’s message?

Labelling mainstream media as “fake news” was common practice throughout Mr Trump’s election campaign and has been a key message in his early presidency.

But according to Melbourne University Election Watch editor James Cahill, Mr Trump’s fascination with fake news is because, before his presidency, the business tycoon only received positive media coverage.

“Trump seems to be genuinely surprised that he’s not covered the way he was in his New York and business world. Back then he was a great source with juicy details in the part-business, part-gossip, part-celebrity world so he generally got positive coverage,” Mr Cahill told The New Daily.

“I think he’s genuinely surprised that this is not happening.”

However, despite Mr Trump’s fake news fixation, he didn’t create it, with Mr Cahill saying mainstream media has been the scapegoat of the Republican party for years.

“Donald Trump did not invent using mainstream media as a foil, this has a long history in the GOP, going back to the original conservative takeover of the Republican party in the lead up to Barry Goldwater’s candidacy in 1964. They even talked about fake polls then,” Mr Cahill said.

“Trump didn’t invent that, he simply just put it on steroids.”

“There is a longstanding tradition of this, Trump’s just used it far more aggressively and his base just loves it. They just eat it up.”

It comes after a Politico poll found 45 per cent of Americans trusted the Trump administration over mainstream media.

Some 42 per cent trusted news outlets, while 10 per cent trusted neither.

“Trump and his supporters are challenging reality with ‘alternative facts’ and it’s working,” Mr Cahill said.

“Nearly 45 per cent of the country is believing this. It’s remarkable.”