A self-styled “Muslim Slayer” who sent fake poison to the Queen with a letter saying “The Clowns R Coming 4 You” has been sentenced to 12 years and six months.
David Parnham, 36, posted similar notes to then PM Theresa May and two bishops.
He also sent “Punish a Muslim Day” hate mail urging people to attack and kill Muslims, the Old Bailey heard.
Parnham, from Lincoln, must serve his sentence in hospital until he is well enough to be transferred to prison.
He admitted 15 offences including soliciting to murder, making hoaxes involving noxious substances and bombs, and sending letters with intent to cause distress.
The charges relate to hundreds of letters penned between June 2016 and June 2018.
Sentencing the IT systems analyst, Judge Anthony Leonard QC said: “You have yet to appreciate the seriousness of what you have done and seem to want to return to the community at the earliest opportunity to live with your parents.”
Parnham failed to appreciate the harm he caused to the Muslim and wider community, which meant there was a greater risk of him reoffending, the judge said.
Judge Leonard said Parnham had been suffering from an autistic spectrum disorder but rejected the suggestion he was psychotic at the time of the offences.
His hoax letter to the Queen triggered a Chemical Biological Radiological Nuclear (CBRN) response, the court heard.
Letters to Mrs May and two bishops, as well as the Home Office, in October 2016, also contained white powder and made an apparent reference to reports of attacks by people dressed as clowns.
Last year, Parnham sent more than 300 letters to mosques and public figures calling for attacks in the street as part of a “Punish a Muslim Day”.
In February 2017, he sent a letter to Berkeley Street Mosque in Hull, which contained a drawing of a sword with a swastika on it cutting someone’s head off with the words “You are going to be slaughtered very soon”.
The author signed off as “Muslim Slayer”.
Det Ch Supt Martin Snowden, head of counter terrorism in the north east, said Parnham’s “abusive, racist and threatening language used in the letters was deeply concerning and created considerable distress which cannot be underestimated”.
Iman Atta, director of anti-Muslim hate crime monitoring organisation TellMama, said there were some in the Muslim communities who “were fearful to go out” because of Parnham’s campaign.
Jenny Hopkins, of the Crown Prosecution Service, described him as “a white supremacist with a particular hatred for Muslims”.
The court had heard the IT specialist sent a fan letter in December 2016 to Dylann Roof, the white supremacist gunman responsible for killing nine black churchgoers in Charleston, South Carolina, thanking him “for opening my eyes”.
Parnham first came to the attention of authorities in July 2016 when seven letters intercepted at Sheffield mail centre were found to contain harmless white powder.
During the two-day sentencing, psychiatrists disagreed on whether he had been psychotic at the time he committed the offences.
By Dominic Casciani, BBC Home Affairs correspondent
David Parnham’s “Punish a Muslim Day” letters did not lead to widespread violence.
But TellMama, an anti-Muslim hate crime charity, said it recorded one school bullying incident it could link to Parnham’s call, though not on the scale he had hoped for.
So, in extreme right-wing terrorism terms, Parnham’s impact was one of spreading fear and consuming a great deal of police resources in tracking him down.
His story is not quite over though. Investigators in the UK and US believe he also sent letters to mosques in North America.
There may be many other victims of his hate mail out there who have yet to come forward.