A knife-obsessed cannabis addict was jailed for life yesterday for the brutal murders of two schoolfriends.

Tom Palmer, 20, will serve a minimum of 18 years after a jury at Reading Crown Court rejected his admission of manslaughter on the grounds of diminished responsibility.

It decided instead that he was responsible for murdering Steven Bayliss, aged 16, and 14 year-old Nuttawut Nadauld, known as T-Wood.

A habitual user of canna-bis and a fan of violent horror movies, Palmer butchered the pair with a hunting knife on a leafy footpath near Wokingham, Berkshire, in September 2005.

After the case, Rethink, a mental health charity, called on the Government to fund a nationwide public health campaign warning people of the risks of cannabis use, especially heavy-strength skunk.

Palmer told prison doctors that he had tried and failed to kick his cannabis habit in the year before the killing. After trying the drug at the age of 14 he was smoking it on a daily basis by the time he reached 15.

He was not smoking on the day of the killings, but told doctors he had been using the skunk form of the drug regularly in the preceding weeks.

Doctors told the court the drug had "exacerbated" Palmer's anxiety and the strange auditory and visual hallucinations he reported suffering in the months before the attacks.

Palmer told clinicians he had attacked the boys when they tried to comfort him as he had a panic attack.

Paul Corry, Rethink's director of public affairs, said: "We now know that cannabis can be a trigger for mental health problems and smoking it under the age of 18 can double people's chances of developing psychosis.

"While it is a terrible tragedy that two young people lost their lives in such a shocking way, homicide involving people with mental health problems is fortunately very rare.

"The Government must invest in a wide scale public health campaign so that young people know cannabis is not risk-free."

The jury were told Palmer's personality appeared to change in the months before the killings. One friend became concerned when he discovered Palmer had used one of his knives to carve swastikas into his chest.

After killing the boys, Palmer called police to say two people had been "cut a little bit" before handing himself in.

Police found the two schoolboys' bloodied bodies in woods. They were lying in the foetal position with their heads so close they were almost touching.

Steven's blue BMX bike was nearby and he was still clutching his rucksack.

Palmer later told officers he carried out the attack after the two boys mocked him about his odd eating habits and his parents' divorce.

The jury heard that Steven and T-Wood were in a group of about 15 friends in Wokingham who would meet in the woods to drink alcohol and smoke cannabis. Palmer would sometimes video them copying stunts from the Jackass television programme.

The court heard Palmer had a fascination with knives, buying the one he killed the boys with from a local sports shop.

Palmer's girlfriend, 17-year-old Ruth Cunningham, said he would watch horror films, often involving martial arts.

In the days before the deaths, Palmer repeatedly watched a DVD about a serial killer who filmed himself stabbing his victims to death, the court heard.

Consultant psychiatrist Dr Robert Ferris, who has been treating Palmer since his arrest, said: "I believe that his state of mind at the time of the killings was not normal. This was exacerbated, but not caused, by cannabis."

A Home Office spokesman later said use of the drug had fallen by almost a quarter since 1988, with the decline largely among young people.

A TV campaign costing £1.8 million had demonstrated how the drug causes damage to the brain and the Government had spent £23 million in recent years on raising public awareness of the dangers, he added.