A full chemical attack alert was triggered after a self-styled “Muslim Slayer” sent fake poison to the Queen with a letter saying: “The Clowns R Coming 4 You”, a court has heard.
David Parnham, 36, sent similar notes to then PM Theresa May and two bishops, the Old Bailey heard on Monday.
He also sent “Punish a Muslim Day” hate mail, urging people to earn points by attacking and killing Muslims.
He has admitted 15 offences, and is due to be sentenced on Tuesday.
The offences relate to hundreds of letters penned between June 2016 and June 2018.
Parnham, from Lincoln, has admitted soliciting to murder, making hoaxes involving noxious substances and bombs, sending letters with intent to cause distress, and encouraging offences.
The hoax letter to the Queen triggered a Chemical Biological Radiological Nuclear (CBRN) response, the court was told.
Royal household staff who handled the mail were quarantined from others for hours while experts raced to identify the substance.
Parnham claimed he did not recall writing to the Queen, the court heard.
The 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.
Parnham also sent letters full of white powder addressed to former prime minister David Cameron, the Tory peer Lord Ahmed of Wimbledon and a number of mosques.
The letter to Mr Cameron contained the sentence “Allah is great”, while letters to MPs and mosques contained the wording “Paki Filth”.
The authorities were alerted to his activities in July 2016 when seven letters were intercepted at Sheffield mail centre and found to contain harmless white powder.
A further 11 letters were found to have been delivered.
‘I felt total shock’
In March 2018, 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”.
Liberal Democrat peer Lord Hussain revealed his “total shock” and “fear” at receiving one of the letters, which had been forwarded from the House of Lords to his home address while he was unwell.
“As I read it for the first time I felt total shock at its contents as well as fear, not only for myself but for my family, my home and all other Muslims,” he wrote in a victim impact statement read out in court.
“I have lived in this country for 47 years and have never before seen or read anything like this,” he added.
In December 2016, Parnham wrote a fan letter to Dylann Roof, a white supremacist who shot nine black parishioners dead 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’.”
The 2018 “Punish A Muslim Day” letters encouraged violence on 3 April – Roof’s birthday.
Letters were also sent to various mosques and Islamic centres in February 2017.
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 included tips on how to kill people and an offer to make a charity donation of £100 for each death.
As he appeared in court for a sentencing hearing, a psychiatrist revealed Parnham did not regret his actions and did not consider them “particularly serious”.
Dr Martin Lock said: “He told me if he went to prison it would be one to two years.”
Although Parnham was on the autistic spectrum, Dr Lock said he was not psychotic, and expressed concern that the defendant had attempted to “mislead” medical professionals.
Parnham, of St Andrew’s Close in Lincoln, was caught through DNA, handwriting and fingerprints on the letters.
He refused to answer any questions when he was arrested in June last year.
Judge Anthony Leonard QC indicated that he would complete sentencing on Tuesday.