JULIA DONALDSON, author of The Gruffalo, the bestselling children's book about a mouse's adventures with a monster, has backed the reclassification of cannabis, blaming the drug for exacerbating the mental illness that drove her son to suicide.

Speaking for the first time about the death of her 25-year-old son Hamish, Donaldson said he had killed himself after a long battle against severe psychiatric illness characterised by psychotic episodes and depression.

His suicide followed a period of heavy drug use, which progressed from cannabis to cocaine.

Donaldson said she supported the government's decision to strengthen penalties for possessing the drug. Under the move, announced last week by Jacqui Smith, the home secretary, the maximum sentence for possessing cannabis will rise from two to five years.

The decision went against the the recommendation of a review by the Advisory Council on the Misuse of Drugs, which concluded that it should remain a class-C drug, despite a probable causal link between cannabis use and psychotic illness.

“I am in favour of the B-grade reclassification for cannabis,” said Donaldson. “I'm very against drugs and I'm against hash.”

The children's author, who lives in Glasgow, added that she did not believe the drug had caused Hamish's illness, but felt it had contributed to his decline.

“Hamish had much too much hash and it was horribly demotivating, apart from anything else. I think it did affect him in the long term,” she said.

Donaldson's comments add to the debate over the dangers of cannabis.

The advisory council concluded that there was a “probable, but weak, causal link between psychotic illness, including schizophrenia, and cannabis use”. However, it found that among the population as a whole, it only played a “modest role” in the development of psychiatric conditions.

Nevertheless, Smith said the government was unwilling to risk young people's health and was concerned about the effects on mental health of smoking high-strength “skunk” cannabis.

The decision to upgrade cannabis was criticised by police, who said the policy would be difficult to enforce and that they would continue to give people carrying a small quantity of the drug a “slap on the wrist”. Under-18s caught with the drug will not be treated any more harshly, to avoid criminalising them.

Since cannabis was downgraded four years ago, the number of dealers sent to jail has fallen to a 10-year low. Prosecutions have fallen by a third.

Last night, Marjorie Wallace, chief executive of Sane, the mental health charity, welcomed Donaldson's intervention and said she believed the government was right to reclassify the drug.

“In more than 20 years of campaigning, seldom have I come across a case of a young man who develops schizophrenia where cannabis has not been involved,” she said.

“It is important that we warn parents, teachers and young people themselves that while many people can take cannabis without any serious problems, those who are vulnerable risk lifelong mental damage.”

Donaldson said she first became aware something was not right with Hamish at an early age. He was repeatedly excluded from school and his behaviour became difficult to manage.

“He wouldn't actually hit me, but he might push me, and I could see that something really dangerous could happen,” said Donaldson.

He was finally diagnosed with schizo-affective disorder, a psychotic illness coupled with mood imbalance. In 2003, Hamish's maternal grandmother died and a short time later a close relative committed suicide. Drunk and psychotic, he assaulted his father, Malcolm, and though charges were later dropped, he was briefly imprisoned.

On the day of his death, about a week after he came out of prison, he had a quarrel with his girlfriend. “I don't know exactly what happened on the last day,” said Donaldson, “But he walked onto the railway line and stood with his back to the train. He was captured on CCTV. He was killed instantly by the blow.”

Donaldson, who has never spoken about the tragedy before, said she had decided to do so to raise the profile of people with mental health problems and their families.

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