A website which glorifies violence and uses imagery suggestive of assassination has launched a paid for online attack ad against Jeremy Corbyn, openDemocracy can reveal.

Despite Facebook’s pledge that there will be transparency around political advertising in the UK election, a paid-for ad branding the Labour leader a “terrorist sympathiser” does not appear in Facebook’s ad library.

The anti-Corbyn advert is promoted by a website called Direct HAF which sells clothing with a mercenary theme. The site’s logo is a skull with a bullet hole in it and it proudly declares itself “comfortable with violence”.

“As General Mattis said 'Be polite, be professional, but have a plan to kill everybody you meet',” the website says.

The DirectHAF Facebook page was created on October 10. Facebook has previously said that its ad library has made UK political adverts “transparent” but the paid-for anti-Corbyn ad does not appear in the social network’s transparency data, which makes it impossible to tell how much money has been spent on it, who has been targeted with this advert, or whether the page has run similar adverts.

The Electoral Commission and experts have raised concerns about so-called “third party” campaigns pushing political ads on social media unchecked.

The DirectHAF ad appears in the context of growing fears about violence against politicians, with the Labour MP Jo Cox being assassinated by a far-right terrorist ahead of the 2016 European Union referendum.

An Oxfordshire woman who alerted openDemocracy to the post, and who wished to remain anonymous, said that she had reported the page to counter-terrorism police, in light of its explicit support for violence, images associated with assasinations, and singling out of a specific public figure.

She said “I was deeply concerned to see Facebook allowing a paid advert from a group which uses imagery directly implying assassination, whilst singling out a specific, high profile politician in an era of growing political violence. And so I reported them to the counter-terror ‘Prevent’ programme.”

Responding to openDemocracy’s research, Iain Overton, Executive Director of Action on Armed Violence, said:

“The notion that the sale of such an ugly smear would support military charities is concerning, and speaks of the way… issues such as nationalism, the veteran and the poppy [have been appropriated].

“It’s part of a wider lurch to the right where words like treachery, terrorist [sympathiser] and Stalinist are being used to denounce anyone who does not agree with their view, and it’s a view seen to be implicitly pro-military, pro-Brexit and pro-intolerance.”

Shaun Boothroyd a Facebook user who also saw the post, said:

“I thought it was disgusting, especially in the light of the events around Jo Cox's murder and the general escalation in abuse and threats of violence aimed at our members of Parliament including my local Labour MP who's office has been defaced with vile, homophobic graffiti on three occasions.”

Direct HAF has a number of T-shirts on their website which glorify violence, with slogans including “happiness is high explosives”, a skull with a bullet shot through it with “head shot”, another with a skull and “kills pay the bills” and one with the image of a semi-automatic shotgun and described as “merc life”.

Another T-shirt quotes Ernest Hemmingway, saying: “There is no hunting like the hunting of man, and those who have hunted armed men long enough, and liked it, never care for anything else thereafter.”

On their website, the business says: “Here at Direct HAF we feel strongly patriotic, we support our Armed Forces and Emergency Services wherever they may be.

“We are a brand forged from the Global War On Terror where our dark sense of humour was honed. While we think you should be comfortable with violence, diplomacy has its place. As General Mattis said 'Be polite, be professional, but have a plan to kill everybody you meet'.

Before signing off with their slogan: “Direct HAF - Comfortable with violence.”

Direct HAF has not responded to openDemocracy’s request for a comment.