A suspicious package sent to a Muslim MP at the Houses of Parliament is feared to be linked to "Punish a Muslim Day".
The vile posters, which the Met police are also investigating, encourage people to get points for committing increasingly serious offences against Muslims.
A member of Huq's staff has been taken to hospital as a precaution after opening the letter, which the MP told the Guardian was a "sticky substance" and the police had described as "low-level noxious".
It follows reports of threatening letters calling for a day of violence against Muslims on April 3. "It is terrible though, it's really very frightening", she told the Guardian.
"Tell MAMA documented previous year how the so-called "Muslim Slayer" targeted mosques in parts of London and the United States in 2017 with similar threats, and in some cases, with non-toxic white powder".
Sources said a mysterious liquid was discovered in Mohammad Yasin's office this lunchtime - but it was later found not to be hazardous.
The people behind this initiative have not been identified yet, but their letters are distinguished by a hateful speech: "They have hurt you".
A parliamentary spokesman confirmed four packages had been sent to the building in recent days and said police were investigating the latest package.
The letters, which are now being investigated by police, encourage people to get points for committing increasingly serious offences against Muslims.
Police were called to the office of Mohammad Yasin at 1pm yesterday and MPs and staff on the first floor of the Norman Shaw North building in Westminster were told to leave the premises or stay shut in their offices.
The substance has since been found to be non-hazardous, according to police.
British police investigated the receipt of a racist letter in a suspicious package to a Muslim lawmaker on Tuesday, one day after two other Muslim lawmakers said they received similar packages.
Two people have been taken to hospital as a precaution after the package was reportedly sent to Labour's Mohammed Yasin, the MP for Bedford. "The incident, which was contained to one room, has been stood down".