A thug whose "almighty blow" to the face of a young man led to him suffering catastrophic brain damage has been jailed for 21 months.

Harry Gregson was critically injured after he was knocked unconscious by a single punch from Mark Storey and struck his head on the concrete ground.

A judge described the attack and 21-year-old Storey’s behaviour afterwards – lying that he acted in self-defence and trying to get others to help him avoid detection – as cowardly.

Judge Martin Joy added: “Those who commit violence in town centres at night can expect to be severely punished. Such violence simply cannot be tolerated. There have to be sentences to deter people acting in this way.”

Mr Gregson, 22, he said was an extremely fit man with a promising future, hoping to join the Royal Navy, who would now only be able to earn the minimum wage for the rest of his life.

His father told in a statement how he feared his son would die after being admitted to hospital. His mother had to give up work as a teacher to look after him.

Maidstone Crown Court heard two groups of people clashed in the street in Tonbridge while moving between bars on Christmas Eve 2016.

Mr Gregson was standing to the side of his group when Storey approached and “without provocation or lawful reason” punched him in the face.

“That punch had appalling consequences because such was the veracity of it, it knocked him out,” said prosecutor Ed Fowler.

“He collapsed to the ground where his head hit the pavement hard. The impact fractured his skull and as a consequence he suffered brain damage.”

Storey, of Hampstead Lane, Nettlestead, had left the scene by the time the police arrived. The force of the punch injured his hand and he went to A&E on Christmas Day for treatment.

Mr Fowler said police retrieved extremely incriminating texts, some of which were deleted, from Storey’s phone. Some involved him telling his girlfriend not to reveal he had hit the victim.

Ellie Dunlop told the jury she was with Storey’s group when she saw him come from the side of her and punch Mr Gregson in the face.

“Harry Gregson stood for a second and fell to the floor,” she said. “He sort of lost his balance. He literally just fell back. I heard a thud.

“Harry had been standing back. I think he was trying to stop it (the trouble). He was doing nothing, from what I saw.

“I sat with Harry to make sure he was OK and to call an ambulance. He was unconscious and not responding.”

Story denied inflicting grievous bodily harm, but was convicted on Monday.

He claimed his brother was being attacked and he was trying to get him out of trouble. He said he was also attacked and he then lashed out.

“I swung my arm out,” he said.

“I felt a connection with my right knuckle. I swung backwards with a clenched fist. I did it because I was scared for my own safety.”

After the guilty verdict, Judge Joy said Mr Gregson had by a single punch been reduced to someone who could “scarcely count to 10”.

He told Storey he had been convicted on overwhelming evidence.

“You plainly didn’t intend to cause him a fractured skull,” he said.

“You did intend to hit him as hard as you could.

“You behaved in a very cowardly way. You fled, lest you be identified as the person who punched him to the ground.

“The effect on the victim was severe. His father has written a most moving impact statement.”

Mr Gregson, 53, said his son remained unconscious until 5am on Christmas Day. After he did come to, he slumped in his arms and was unresponsive.

“It is quite plain they knew his recovery was in the lap of the gods,” said Judge Joy.

“It was a living hell for them. They thought he might die.”

The judge added: “It is a very familiar picture in town centres in this country. It is a real concern and must be stamped out.

“There has to be a clear and consistent message that such offences endanger safety of decent law-abiding citizens.”