Home Secretary Yvette Cooper has written to Elon Musk's X, Mark Zuckerberg's Meta, TikTok, Google and YouTube calling on them to "urgently review" material accessed by killer Axel Rudakubana which is still available online.
In a joint letter with Technology Secretary Peter Kyle, first reported by the Telegraph and seen by the PA news agency, Ms Cooper said the ease with which "such dangerous and illegal" content can be viewed was "unacceptable".
Rudakubana, 18, was jailed for life with a minimum term of 52 years earlier this week after pleading guilty to murdering Alice da Silva Aguiar, nine, Bebe King, six, and Elsie Dot Stancombe, seven, at a dance class in Southport last July.
He also admitted attempting to murder eight other children and two adults, possession of a knife, production of a biological toxin, ricin, and possessing information likely to be useful to a person committing or preparing to commit an act of terrorism.
Before leaving home, he had searched online for "Mar Mari Emmanuel stabbing" - the knife attack on a bishop in Sydney, Australia, in April 2024.
The graphic video was removed in Australia but is still available to view in the UK, the cabinet ministers said.
"We are therefore urgently asking you to ensure this is also removed from your platforms to ensure that it is not used to inspire other attacks," they wrote.
"This trial has laid bare the potential consequences of failing to act on such content.
"We would now urge you to urgently review again the specific content accessed by Rudakubana and the options available to you to remove all instances of this specific material, found to be materially relevant to this tragic case, present on your services."
Rudakubana was jailed for life after pleading guilty to the murder of three girls at a dance class. (AP PHOTO)
Rudakubana also had a PDF file entitled Military Studies In The Jihad Against The Tyrants, The Al Qaeda Training Manual, which led to him facing the charge under Section 58 of the Terrorism Act 2000.
The ministers warned that the killer had been able to easily obtain this document online and it "continues to remain available".
"We must prevent further unlawful access to instructional material by those wishing to do harm to the United Kingdom," they said.
"We are again requesting that you, as industry leaders, swiftly remove any unlawful material on this list available on your services, including the material used by Axel Rudakubana."
Under the UK's Online Safety Act, from March platforms will be required to remove illegal content, including violent material.
But the home secretary and technology secretary said services have "a moral responsibility to act now".
"There is no justification for waiting for laws to kick in," they said.
Police have warned it could take years to uncover what was in the killer's internet browsing history, which he deleted before he left the house to carry out the atrocity.
To obtain the information, detectives needed to go through US companies Microsoft, which owns the search engine Bing, and Google, which owns the web browser Chrome.
According to Google, the police could have applied through the UK courts for an overseas production order which, if granted, would have meant the data was produced more quickly.
A spokesman said earlier this week the company was in touch with Merseyside Police to support their inquiries and that its "deepest sympathies are with all of the families and individuals affected by this horrific attack".
The force said requests had been submitted through the correct channels for a criminal investigation.
Microsoft, Google, YouTube, TikTok, Meta and X have been contacted for comment.
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