A man with autism was shot with a stun gun by police and charged with assaulting an officer but the case collapsed after his mother found CCTV footage of the incident that undermined the prosecution’s case.

The man from Bristol was taken to court and could have been jailed if he had been convicted but the Crown Prosecution Service (CPS) dropped the case after footage emerged contradicting police claims that the man pushed an officer.

Avon and Somerset police insisted the use of the stun gun and the arrest were reasonable but confirmed that an officer had been dealt with for not producing the footage.

The police watchdog, the Independent Office for Police Conduct (IOPC), criticised the officer over the failure to gather the CCTV evidence and recommended further disability awareness training.

Officers were called to The Laurels, a supported housing unit for vulnerable adults, in August 2015 after reports of a cracked window.

The call was graded as requiring an immediate response and officers attended the scene and used a stun gun during the arrest of the man, who was 24 and has not been named. Police claimed he pushed one of them “double-handed in the chest with open palms”, causing him to stumble back two feet.

By the time the case reached court, the man’s mother had obtained the CCTV footage and the prosecution was halted in November 2015 with the CPS deciding it showed a different version of events to that given by the police.

The mother has called for the use of Tasers to be investigated. She said: “If [my son] had been found guilty on the basis of their evidence, he would have been sent to prison.”

The IOPC carried out an investigation and cleared the police officers involved in the arrest of wrongdoing. However, it did criticise another officer over the failure to produce the CCTV footage.

The Bristol East MP, Kerry McCarthy, said the man was talking to the police “like a child” and she did not understand why he had been shot.

She said: “”There are questions about the way they behaved when they turned up, questions about why the use of Taser, and then where did this charge of assaulting a police officer come from. The charges came out of nowhere. There’s the whole issue about why they didn’t look for CCTV and why the mum had to do that. She was dogged and persistent and thank goodness she was.”

Avon and Somerset police defended the officers involved in the original incident.

They said that the 999 call made to the police said a man was “terrorising other clients” and “intoxicated” and had “broke a window in one of the client’s doors”.

The force said the incident was subject to a detailed and robust two-year investigation by the IOPC, which found there was no case to answer for misconduct for any of the officers involved.

A spokesperson said: “The decision to arrest the man for criminal damage and assaulting an officer was necessary and the report stated both the arrest and use of Taser were reasonable in the circumstances. There was no evidence officers used any more force than they needed to.”