An inmate accused of killing another prisoner at Lincoln Prison was cleared of manslaughter by a jury at Lincoln Crown Court.
The jury spent four hours deliberating at the end of the trial of Emmanuel Dos Santos before returning a verdict of not guilty.
Dos Santos was alleged to have killed Samuel Harrison with a single punch in a shower area at the prison.
Harrison, 29, fell on to a tiled floor and suffered severe head injuries.
He was taken to Lincoln County Hospital. He died two weeks later without regaining consciousness.
Dos Santos told the jury that he acted in self defence.
He told the jury that Samuel Harrison was “spitting angry” when he approached him on the jail’s B wing.
Dos Santos said ” I was trying to remonstrate with him. He comes towards me. He had a cup in his hand. I didn’t know what was in it. In jail people swill people with hot water.
“That’s when I struck out. It was defensive. I punched him once. It was just like a defensive punch to get him away from me because he was getting close to me.
“He went straight back, fell and banged his head. I went over and tried picking him up and put him back down. I panicked then and rushed off. There were two lads outside. I said to them to look after him.”
Dos Santos, who was in prison after being recalled to serve part of an earlier sentence, admitted that when he was interviewed by police he had claimed Samuel Harrison had been in a zombie-like state and simply fell pole-axed to the floor.
Dos Santos told officers he believed Harrison was “a druggie” who was using the synthetic drug mamba at the time.
But he told the jury that he lied during the police interview out of panic.
Emmanuel Dos Santos , 43, denied the manslaughter of Samuel Harrison in September 2017.
Jonathon Dee, prosecuting, earlier described the case as a “classic one punch manslaughter”.
He said “On 11 September 2017 Mr Dos Santos punched Samuel Harrison.
“That blow would not have killed him. Mr Harrison as a result of that punch fell back.
“When he fell back he hit his head on the floor, that is what killed him.”
Mr Dee said the prosecution did not claim that Dos Santos intended to kill or seriously injure Mr Harrison.
“He did an unlawful act. He punched him and that ultimately caused his death.
“The only issue is whether the punch was lawful, ” said Mr Dee.