David Parnham admitting sending the letters after he was arrested by counter-terrorism police.
A self-styled “Muslim Slayer” who sent the Queen fake anthrax with a note saying “The Clowns R coming 4 you” has been locked up for 12 and a half years.
White supremacist David Parnham, 36, wrote to prominent figures including the Queen and former prime ministers Theresa May and David Cameron as part of a two-year hate campaign.
The IT systems analyst also caused widespread fear and upset through “Punish A Muslim Day” letters, encouraging violence in the community, the Old Bailey heard.
He tried to instil further alarm by posting white powder in the hope it would be mistaken for anthrax, the court heard.
When the Queen was sent an envelope containing the substance, a chemical, biological, radiological and nuclear (CBRN) response was launched.
Members of the royal household were kept separate from other staff and became “anxious for their health” and the safety of colleagues, the court heard.
Parnham pleaded guilty to 15 offences relating to hundreds of letters written between June 2016 and June 2018.
The charges included encouraging murder, making hoaxes involving noxious substances and bombs, sending letters with intent to cause distress, and encouraging offences.
Judge Anthony Leonard QC said Parnham had been suffering from an autistic spectrum disorder but rejected the suggestion he was psychotic at the time of the offences.
He sentenced Parnham to 12 years and six months in custody to be served in hospital until he is well enough to be transferred to prison.
Judge Leonard told Parnham: “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’s failure to appreciate the harm he caused to the Muslim and wider community meant the risk of reoffending was greater, the judge said.
The court heard that Parnham’s activities first came to the attention of authorities in July 2016 when seven letters were intercepted at a Sheffield mail centre and found to contain harmless white powder.
A further 11 letters were identified as having been delivered.
A letter to Cameron contained the wording “Allah is great”, while letters to MPs and mosques contained strong racist language.
In October 2016, more letters containing white powder said “The Clowns R coming 4 you” and were intended to reach the Queen and May.
In December 2016, Parnham sent a fan letter to Dylann Roof, the white supremacist gunman responsible for killing nine black church goers in Charleston, South Carolina.
He told Roof: “I just wanted to thank you for opening my eyes. Ever since you carried out what I’d call the ‘cleansing’ I’ve felt differently about what you’d call ‘racial awareness’.”
In February 2007, letters were sent to mosques and Islamic centres around the UK.
A letter to Berkeley Street Mosque in Hull 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”.
In March 2017, letters were sent to addresses around the University of Sheffield campus calling for the extermination of minority racial and religious groups.
They contained suggestions on how to kill people and an offer to make a donation of £100 to charity for each death.
In 2018, the series of typed “Punish A Muslim Day” letters were sent to a large number of people, encouraging violence on April 3 2018 – Roof’s birthday.
Parnham, of St Andrew’s Close in Lincoln, was caught through DNA, handwriting and fingerprints on the letters.
Psychiatrists disagreed on whether he had been psychotic at the time he committed the offences.
Dr Martin Lock expressed concern that the defendant had attempted to “mislead” medical professionals.
He told the court Parnham felt “disgusted and ashamed” of what he had done but did not regard it as very “serious”.
Parnham told Dr Lock: “I just wrote letters, I did not mean for anyone to feel fear.”
Dr Paul Wallang said Parnham was suffering a psychotic illness and had felt “paranoia and suspiciousness”, particularly towards religious groups and prominent individuals.
However, he conceded it was possible Parnham could have “pulled he wool” over the eyes of medical professionals dealing with his case.