Starmer's spokesperson said Monday there was "no justification" for the comment.
A UK government minister on Tuesday slammed American tech billionaire Elon Musk for his "unacceptable" comments on social media about far-right riots gripping England and Northern Ireland.
The owner of X drew criticism for writing on the site on Sunday that a British "civil war" was inevitable. He sparked further ire on Monday with a provocative reply to a tweet by UK Prime Minister Keir Starmer.
"Use of language such as a 'civil war' is in no way acceptable," said justice minister Heidi Alexander, branding Musk's comments "deeply irresponsible".
"We are seeing police officers being seriously injured, buildings set alight, and so I really do think that everyone who has a platform should be exercising their power responsibly," she told Times Radio.
Riots by anti-immigrant demonstrators have spread across England following the murder of three children at a Taylor Swift-themed dance party in Southport, northwest England, on Monday last week.
They have been fuelled by online disinformation that the stabbing suspect was a Muslim asylum seeker. He is 17-year-old Axel Rudakubana, who was born in Britain.
The rioters have targeted mosques and hotels housing asylum seekers, forcing the government to provide emergency security to Islamic places of worship.
In a post on X on Monday, Starmer vowed to apply "criminal law online as well as offline", adding that "we will not tolerate attacks on mosques or on Muslim communities".
Musk replied: "Shouldn't you be concerned about attacks on *all* communities?"
'No justification'
His original "civil war" post came in a reply to another X user blaming the riots on "the effects of mass migration and open borders".
Starmer's spokesperson said Monday there was "no justification" for the comment.
Influencer Andrew Tate and far-right, anti-Islam figurehead Tommy Robinson are among people promoting false claims about Rudakubana on X.
Starmer's spokesperson also blamed "bot activity online", suggesting that the false rumours could have been "amplified with the involvement of state actors".
EuropeInvasion, an anti-immigrant X account with hundreds of thousands of followers, still has a post up falsely claiming that the attacker was "confirmed to be Muslim".
An AI-generated image depicting Muslim men chasing a child wearing a British flag has over 900,000 views.
Technology minister Peter Kyle met representatives from TikTok, Meta, Google, and X on Monday and warned that social media users spreading misinformation will have "nowhere to hide".
Musk -- who has reduced content moderation on the platform since taking over Twitter, instead relying on user-written "community notes" -- regularly voices support for right-wing causes and politicians like Donald Trump in the US and Javier Milei in Argentina.
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)