The son of President-elect Donald Trump’s national security adviser is spreading a fake news story that claims a Washington DC pizza shop is a front for a child sex ring led by Hillary Clinton.

On Sunday, the son of retired Lt Gen Michael Flynn, Michael Flynn Jr, tweeted, “Until #Pizzagate proven to be false, it'll remain a story. The left seems to forget #PodestaEmails and the many ‘coincidences’ tied to it.”

It comes as 28-year-old Edgar Maddison Welch fired a gun in the Comet Ping Pong restaurant to allegedly “self-investigate 'Pizza Gate'”, described by police as a "fictitious conspiracy theory".

Originating on alternative message board 4chan, supporters of the theory believe emails hacked from the Democratic Party and leaked by Wikileaks prove a sinister connection between restaurant owner James Alefantis and Ms Clinton.

However, the story has been proven to be completely fabricated, and while Mr Alefantis admits he has some prominent Democratic friends in Washington, he denies ever meeting Ms Clinton.

The conspiracy centres around a child sex ring being run out of Mr Alefantis’ basement, which he says is discredited by the most basic of factual errors.

"They ignore basic truths. We don't even have a basement,” he has said.

"Sometimes an innocent picture of a child in a basket is just an innocent picture of a child in a basket and not proof of a child sex trafficking ring.”

Allies of Mr Flynn jr, including Citizens for Trump’s special projects director, Jack Posobiec, have stood by the theory despite it being widely debunked.

“False flag. Planted Comet Pizza Gunman will be used to push for censorship of independent news sources that are not corporate owned,” he tweeted on Sunday.

Mr Flynn jr also retweeted “confirmation” from Mr Posobiec that Welch, the gunman who stormed the restaurant, was an actor.

For weeks, Comet Ping Pong has been plagued with messages of abuse surrounding the false allegations, worsened by the #PizzaGate hashtag gaining widespread traction on social media.

Last week, it announced it would be implementing increased security measures at shows hosted by the restaurant.

“Comet Ping Pong, like any respectable venue, is dedicated to creating a safe and inviting space for all of our concert-goers,” a statement on the restaurant’s Facebook page reads.

“There have been no hostile situations at the venue, and we do not anticipate any altercations as much of the harassment has occurred online, but as a precaution we now have security and police present at every show.”

This is not the first time Mr Flynn Jr has spread conspiratorial rhetoric via social media.

He also alleged top Hillary Clinton aide Huma Abedin had a connection to the Muslim Brotherhood and pushed a conspiracy theory that Sen Marco Rubio was a closeted homosexual who abused cocaine.