A schoolboy has been sentenced to four years in custody after he set off a homemade bomb in a Sainsbury’s supermarket.

The 16-year-old, who has not been named, wedged the explosive into a toilet at the store in Blackheath near Birmingham.

He had made the device at home following step-by-step instructions he had found online.

Sentencing yesterday, Judge James Burbidge QC said that the boy did not follow any extremist groups and is ‘not a terrorist’. He said:

"This was not a random act but a prepared act. You did not intentionally want to cause harm but it was such a highly reckless act that it was fortunate people were not caused harm."

The boy wore a bandana and covered the lower part of his face and nose when he entered the supermarket on May 27. He then went into the toilets, where he placed the bomb into a cubicle.

The court heard that he later crouched over a cleaner with a blade in a bid to warn her away after she spotted him the the toilet. She ran and raised the alarm with other colleagues after spotting smoke rising from the toilet bowl.

The teenager then fled the scene, ripping off his disguise in a bid to blend in with shoppers – who were being evacuated by staff. Armed police were called to the scene and the area was cordoned off.

The schoolboy was later arrested and reportedly told police her had spent a month visiting anarchist websites in a bid to learn how to make a bomb.

He said he had not been forced to make it, but could not explain why he targeted the supermarket.

The court heard that the explosion caused more than £1,000 worth of damage to the store, which also lost £110,000 in revenue as a result of being closed for the day. Defence barrister Mr Andrew Jackson said:

"This whole dreadful episode appears to be motivated by a combination or curiosity, and almost addiction, to computers, and frankly wanting to see what would happen."

The 16-year-old pleaded guilty to making an explosive substance for unlawful purposes and doing an act with intent to cause explosion likely to endanger life or injure property.

He also admitted possession of a bladed article in a public place. The schoolboy was sentenced to four years and four months in custody.