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A London teenager has been found guilty of stabbing a police dog and an officer in a dramatic chase following a suspected robbery.

The boy, 16, who cannot be named because of his age, knifed German Shepherd dog Finn with a 12-inch hunting blade after he was chased through the streets of Stevenage in the early hours of October 5 last year.

The teenager admitted possessing a knife and imitation firearm but claimed he was acting in self-defence after fearing for his safety and being “bitten up” by the animal. He said he only fled from police because he was worried about being caught with the blade.

Police dog handler David Wardell broke down in tears at Stevenage Youth Court as he described the moment the suspect brandished “the largest knife” he had ever seen.

But district judge Jo Matson found the boy guilty of actual bodily harm, as well as criminal damage to the dog, and warned he could face jail when he was sentenced next month.

Following the one-day trial, the judge said the defendant's evidence was "not credible or truthful".

She added: "His evidence does not add up. I do not accept that he found it necessary to take the actions he did to defend himself.

"He put himself in a position where it was necessary for Finn the police dog to stop him from running away with a knife in his hand.

"Stabbing a dog you know to be a police dog and lunging a knife at a police officer were not necessary or proportionate and I do not find he (the defendant) believed them to be at the time."

Finn the dog required a four-hour operation, including having two sections of his lung removed, after being stabbed in the body and head. The court heard the knife narrowly missed the dog's heart.

The judge added: "PC Wardell gave very emotional evidence today and was clearly very distressed by what happened that day, and still is."

Addressing the youth, who had his mother by his side at court, the district judge said: "All sentencing options remain open, including custody."

He returned to the force after making a recovery and has subsequently retired from service, Hertfordshire Police said.

Recalling the incident, PC Wardell said: "It was the largest knife I've ever seen.

"Finn didn't let go at all of the suspect.

"I was in fear of my life, and in fear of Finn dying in front of me.

"Finn is one of the highest trained dogs in the police force, I wished neither Finn nor myself were in that garden, but we were."

The youth will be sentenced in Bromley, south London, next month.

Additional reporting by Press Association.