LONDON (REUTERS) - The mother of a man charged with trying to attack Donald Trump with a policeman's gun at a rally in Las Vegas has pleaded for US officials to send her son back to Britain to have a psychiatric assessment.

Twenty-year-old Michael Sandford was arrested after trying to grab a police officer's side arm at a Trump rally in Las Vegas on June 18.

When asked why, he told police, "To shoot and kill Trump", according to US court papers.

"We want to try to bring him home to the UK, to a secure place that will help him," Ms Lynne Sandford told a news conference in London.

She told reporters that her son suffers from OCD, Aspergers syndrome and depression and was on suicide watch in a Nevada jail.

"Michael is an autistic man who will not have foreseen the consequences of his actions. Autistic adults can have the insight and behaviour of a small child. I accept that Michael has tried to do a bad thing, but he is mentally ill and is not a bad or dangerous person."

The family has set up a crowdfunding campaign online to try to pay for his legal costs and a psychiatric evaluation.

She has so far been unable to travel to America to see her son, who is only able to speak to visitors using a video link.

Sandford's lawyer Saimo Chahal said he is set to face trial in the US on Aug 22.