Britain’s counter-terrorism chief has said far-right terrorists are being radicalised by mainstream newspaper coverage, while also criticising the hypocrisy of outlets such as Mail Online, which uploaded the “manifesto” of the gunman in the Christchurch terror attack.

Neil Basu, one of Britain’s top police officers, said it was ironic that while newspapers have repeatedly criticised the likes of Facebook and Google for hosting extremist content, sites including the Sun and the Mirror rushed to upload clips of footage filmed by the gunman as he attacked two mosques in New Zealand.

“The same media companies who have lambasted social media platforms for not acting fast enough to remove extremist content are simultaneously publishing uncensored Daesh [Islamic State] propaganda on their websites, or make the rambling ‘manifestos’ of crazed killers available for download,” Basu said in an open letter to the media on how to report terrorism.

He appeared to be singling out Mail Online, which uploaded the New Zealand’s terrorist’s 74-page “manifesto” to its website and made the document, which included an explanation of his far-right ideology, available for users to download from one of the world’s biggest news outlets.

Basu, whose job has been described as the toughest in UK policing, said it was time to accept that many terrorists were being radicalised by mainstream news outlets: “The reality is that every terrorist we have dealt with has sought inspiration from the propaganda of others, and when they can’t find it on Facebook, YouTube, Telegram or Twitter they only have to turn on the TV, read the paper or go to one of a myriad of mainstream media websites struggling to compete with those platforms.”

He cited the 2017 terror attack in Finsbury Park in London as an example of where a man was “driven to an act of terror by far-right messaging he found mostly on mainstream media”.

Basu invited national newspaper editors to debate their coverage with “survivors of terrorism and those of us trying to counter it”. He said he hoped the government would deal with the issue of mainstream news outlets amplifying terrorist messaging in its forthcoming proposals on online harms and not just target large social networks such as Facebook and YouTube.

“A piece of extremist propaganda might reach tens of thousands of people naturally through their own channels or networks, but the moment a national newspaper publishes it in full then it has a potential reach of tens of millions. We must recognise this as harmful to our society and security.

“Anyone who seeks to deny the negative effects that promoting terrorist propaganda can have should think carefully about the massive global effort to remove terrorist content from social media platforms and the pressure that governments, law enforcement and, ironically, the mass media has put on those companies to cleanse their sites.”

Mail Online and the Mirror later removed video of the attack, although a spokesperson for the Sun defended using clips on the basis it would “shed light on this barbarous attack and the twisted ‘motive’ behind it”. Both the Daily Mail and the Sun then used their print editions to criticise Facebook’s role in the Christchurch attack, despite uploading clips of the gunman’s footage to their websites.

Basu said all online outlets should also try to deal with the “torrent of hate and abuse below a criminal threshold” that follows any act of terror, amid fears this is driving people off the internet and “creating a permissive environment capable of pushing the most extreme ideologues over the edge”.

“Society needs to look carefully at itself. We cannot simply hide behind the mantra of freedom of speech. That freedom is not an absolute right, it is not the freedom to cause harm – that is why our hate speech legislation exists.”