The suspect accused of murdering three girls in Southport is facing new charges of possessing terrorist material and producing the highly toxic poison ricin, police have announced.

Axel Rudakubana, 18, will appear at Westminster magistrates court on Wednesday charged with producing the biological toxin and having a document titled “Military studies in the Jihad against the Tyrants – the al-Qaida training manual”.

Serena Kennedy, the chief constable of Merseyside police, said the murder of the three girls – Alice Dasilva Aguiar, nine, Bebe King, six, and Elsie Dot Stancombe, seven – was not being treated as a terrorist incident. She said no evidence pointing to a terrorist motive had been discovered.

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Ricin was discovered at Rudakubana’s home in the village of Banks, about 5 miles from Southport, in early August, days after the attack. Kennedy said there was no evidence that ricin was present at Hart Space, the scene of the knife attack, and that counter-terrorism police had “not declared the events of 29 July as a terrorist incident”.

She said: “At this time, counter-terrorism policing has not declared the attack on Monday 29 July a terrorist incident. I recognise that the new charges may lead to speculation.

“The matter for which Axel Rudakubana has been charged with under the Terrorism Act does not require motive to be established. For a matter to be declared a terrorist incident, motivation would need to be established.

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The Guardian understands 2,000 riot officers are on standby on Tuesday, fearing the disclosure of the new charges could trigger potential unrest. Robert Jenrick and Kemi Badenoch, who are days away from learning which of them will become the new Conservative party leader, immediately issued statements raising concerns about the approach of the police and Labour government.

Jenrick said the attack was of “immense public concern” and that people “had a right to know the truth straight away” but that he was “seriously concerned that facts may have been withheld”.

He added: “Any suggestion of a cover-up will permanently damage public trust in whether we’re being told the truth about crime in our country. Keir Starmer must urgently explain to the country what he knew about the Southport attack and when he learned it.

“Across the board the hard reality of mass migration is being covered up. We need the truth – and we need to change.”

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Neil Basu, the former head of the UK’s counter-terrorism policing, also condemned Jenrick, saying such comments might jeopardise justice. “This is irresponsible, and repeating the mistakes of others, failing to calm a very volatile situation which we faced this summer.

“I think he is stupid, he’s trying to win an election, and he is trying to whip up support among his base. He has made a stupid mistake.”