Bereaved families whose loved ones were the victims of an online supplier of suicide kits say they feel insulted by a decision not to prosecute him in the UK.

Kenneth Law was due to appear in court on Friday in Ontario, Canada, accused of selling 1,200 suicide packages across 40 countries, including the UK.

He is expected to plead guilty to charges of aiding suicide and to admit sending products internationally in the knowledge they were likely to be used to end lives. Last month his lawyers confirmed a plea agreement under which previous murder charges would be withdrawn.

An investigation by the National Crime Agency (NCA) into Canadian websites found that 286 individuals had received packages in the UK, leading to 112 deaths.

A day before Law’s court appearance, the NCA and the Crown Prosecution Service told bereaved families they would not be seeking to extradite the 60-year-old to the UK after legal proceedings in Canada had concluded.

Adele Zeynep Walton, the sister of 21-year-old Aimee, from Southampton, who died in 2022 after buying one of the suicide kits from Law’s website, said: “It’s absolutely insane that the NCA and CPS are not going to do anything about it. It is so insulting.”

In a letter to the families, the NCA and the CPS said: “After careful assessment, we agreed that Mr Law should be sentenced for the full extent of his offending within a single sentencing process in Canada. This approach is not unusual in cases involving serious offending that crosses international borders.

“We recognise that this may be painful to hear, and that some victims and bereaved families may have hoped to see a separate prosecution in England and Wales.”

Walton said the scale and novelty of Law’s alleged crimes warranted intervention from the UK authorities. She said: “When have we ever had people using the internet in order to target and seek out vulnerable people and assist them systematically in suicide? This is a new epidemic of assisted suicide. I think the approach of the NCA and CPS needs to adapt with the times.

“If they’re not going to prosecute a man who potentially could be one of UK’s biggest serial killers, then what message does that send to other people like him?”

Walton added: “There are more victims than the Grenfell disaster and yet no one seems to be talking about this and there’s no willingness from the government, from the NCA and CPS to bring justice to families.”

David Parfett, the father of Thomas Parfett, 22, a philosophy student who took his own life in Sunbury-on-Thames, Surrey, said: “I am angry, but I am not surprised. For months, we have been told that the system is working and that existing measures are enough. They are not.

“If our own country will not put anyone on trial for these deaths, the very least it can do is hold a proper inquiry into how they were allowed to happen.”

Last month the government rejected calls for a public inquiry into the issue.

Andy Burrows, the chief executive of the Molly Rose Foundation, said: “Bereaved families have been campaigning tirelessly to hold Kenneth Law to account in the UK and to be told he will not be prosecuted here on the eve of his court case in Canada is a bitter blow.

“Families up and down the country have been impacted by Law’s crimes and should have the right to full justice in the UK. As long as the pro-suicide forum remains online, and while this substance is available in the UK and across borders, more vulnerable people are at risk.”

Next week the bereaved families will meet their lawyers at Leigh Day to discuss next steps. Walton said: “We are going to keep fighting because the only reason that we continue to share our trauma is to prevent future deaths.”

In a joint statement, Joanne Jakymec, the chief crown prosecutor for the CPS, and Craig Turner, a deputy director at the NCA, said: “No outcome in any court can remove the pain victims and their families have suffered. Victims have remained our priority when making decisions to deliver justice.”

Andrew Hudson, a specialist prosecutor at the CPS, defended the decision not to seek Law’s extradition. He said: “If an extradition request was declined, Kenneth Law would never have been required to face justice for the harm caused to victims here. Even if we had succeeded in extraditing Law, a prosecution in England could have been blocked under double jeopardy principles, because the same conduct would already have been punished by the Canadian courts.”

Under an agreed statement of facts, the court in Canada will document that Law sent packages to 286 recipients in the UK, and that 79 deaths in the UK were attributed to products supplied by him.

Hudson said: “Kenneth Law is a serial offender who callously exploited many vulnerable and innocent people exchanging their lives for his financial gain. The approach we have agreed will guarantee justice for our victims in the quickest possible way.”

• In the UK and Ireland, Samaritans can be contacted on freephone 116 123, or email jo@samaritans.org or jo@samaritans.ie. In the US, you can call or text the National Suicide Prevention Lifeline on 988, chat on 988lifeline.org, or text HOME to 741741 to connect with a crisis counselor. In Australia, the crisis support service Lifeline is 13 11 14. Other international helplines can be found at befrienders.org