Counter terror police are investigating the letters, treating them as a possible hate crime.
Bangladeshi-origin MPs Rushanara Ali and Rupa Huq and their Pakistani-origin colleagues Afzal Khan and Mohammad Yasin are the four Labour party MPs to receive the same kind of package in their parliamentary offices within the Palace of Westminster over the last two days.
The substance has since been found to be non-hazardous, according to police.
The letter declared that April 3 would be "Punish a Muslim Day" where points would be given for various acts of violence: 25 points for pulling off a woman's headscarf, 50 points for an acid attack, 500 points for murdering a Muslim and 1,000 for bombing a mosque.
"The letter calling for attacks on Muslim is just the latest example of rising Islamaphobia and deplorable hate crimes in this country", said Pakistani-origin Labour MP Yasmin Qureshi, who raised an urgent question in the House of Commons yesterday calling on the United Kingdom government to do more to tackle such "incitement to violence and help prevent the growth of such extreme views".
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'We assessed two people at the scene and took them to hospital'.
It comes a day after ministers urged members of the public to hand over to police letters which encourage people to "butcher" Muslims.
Huq told the BBC the package sent to her on Monday contained a "low-level noxious substance and an offensive letter".
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.
"[It is] totally unacceptable for Muslim MPs to face this", she said.
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The images on the letters suggest they may have been sent by mysterious "Last Soldiers of Christ" group.
"I have been contacted by a number of people who are anxious about the racist literature that was sent to my office today", Yasin wrote on Facebook.
Ms. Atkins said that letters such as those sent at the weekend would be treated with the "utmost seriousness". "One person attended hospital as a precaution".
Labour's Yasmin Qureshi asked an urgent question in parliament on Monday on the racist letters.
The organization said that some of the letters appear to have been mailed out of Sheffield, the NYT reported.
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