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 ricin 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.

Rudakubana was arrested on 29 July shortly after a knife attack at a Taylor Swift-themed dance class in Southport in the first week of the school summer holiday.

As well as the three murder charges, he is also charged with 10 attempted murders for alleged attacks on a further eight children and two adults. He is also charged with possessing a bladed article.

At a press conference at Merseyside police headquarters on Tuesday, Kennedy said the new charges followed a “lengthy and complex” three-month investigation, and she pleaded with people not to speculate about the alleged offences.

The ricin, a potentially lethal toxin, was discovered at Rudakubana’s home in the village of Banks, about 5 miles from Southport, in early August, days after the attack.

She 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”.

Kennedy 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.”

She added that counter-terrorism officers were continuing to assess the evidence and that the full case against Rudakubana, who was born in Cardiff, would be disclosed at the criminal trial, planned for January.

“I would strongly advise everyone to avoid speculation about the motivation in this case,” she said. “The criminal proceedings against Axel Rudakubana are live and he has a right to a fair trial. It is extremely important that there is no reporting, commentary or sharing of information online that could prejudice these procedures.”

She added: “My plea is to be patient. Don’t engage in rumour speculation. Don’t believe everything you read on social media. We must not lose sight of the families of Elsie, Bebe and Alice, who are still grieving, and the families of the others who were present at Hart Street on 29 July. We must do everything to ensure the justice process is not prejudiced.”

The Guardian understands police had 2,000 officers on standby on Tuesday, fearing the disclosure of the new charges could trigger potential unrest.

The two Conservative leadership candidates, Robert Jenrick and Kemi Badenoch, who are days away from learning will become leader of the opposition, 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.”

Nazir Afzal, the former chief crown prosecutor for north-west England, condemned Jenrick for his comments.

He told the Guardian: “The kind of comment Mr Jenrick has made when proceedings are active, any statements that could prejudice proceedings are unacceptable and may be unlawful.

“There is no excuse for the kind of comments that are being made while proceedings are active.”

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.”

Badenoch said it was “quite clear that there are serious questions to be asked of the police, the CPS and also of Keir Starmer’s response to the whole situation”.

She added: “Parliament is the right place for this to happen. While we must abide by the rules of contempt of court and not prejudice this case it is important that there is appropriate scrutiny.”

Downing Street denied the government had been involved in decision-making about the timing of an announcement about charges against the Southport attack suspect.

Asked if there was any government involvement in decisions on the timing, a No 10 spokesperson said: “No, charging decisions and when those are made are for the CPS (Crown Prosecution Service).”

She also said it was “not correct” to say the government had been involved in withholding facts from the public.

She added: “Charging decisions are independently made by the CPS and I would point you to the CPS statement and statements from the police.”

Dominic Grieve, the Conservative former attorney general, warned that potentially prejudicial comments risked ensuring justice for the victims and their families.

He told the Guardian: “No one should comment on a subjudice case in a manner that prejudices the outcome.

“If a fair trial process is affected by comment in breach of the Contempt of Court Act the rights of the victims are potentially as undermined as the rights of defendants.”

One former Conservative minister said: “I wish they had avoided the temptation to comment ...

“In this case, the police have explicitly warned against jumping to conclusions on the basis of the new charges. And surely the overriding objective be to avoid any risk that trial collapses due to prejudicial public commentary.”

The killings led to the spread of disinformation about the suspect, and to a series of riots across England that resulted in more than 1,000 people being charged.

Since the initial arrest and charging of Rudakubana, the investigation has continued, involving officers from Merseyside police assisted by counter-terrorism detectives.

Dr Renu Bindra, of the UK Health Security Agency, said its experts had found no evidence that any of those present at Hart Space, first responders, or members of the public were exposed to ricin. She said its experts assessed the risk to the wider public as “low”.

She added: “I appreciate that this information may be concerning. However, I would like to emphasise that there has been no evidence of ricin poisoning in anyone involved or connected to this incident.”

After Wednesday’s magistrates court hearing, Rudakubana is next due in court on 13 November at Liverpool crown court.

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