Tory MP Michael Fabricant said politicians fear the letter was linked to the "Punish a Muslim day" hate letters, which have been distributed to homes throughout the UK.
"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".
"There is a definite pattern", she said, after two other Muslim lawmakers reported receiving similar packages on Monday.
The letter has reportedly been received in locations including Bradford, Leicester, London, Cardiff and Sheffield.
"It is to put the frighteners on us but we won't be cowed".
Manchester Gorton Labour MP Afzal Khan was one of four Muslim MP's to receive suspicious packages at Westminster.
"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. Among my colleagues, people have shown support but they are outraged".More news: Matt Damon and Ben Affleck's Production Company Will Adopt "Inclusion Riders"
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Emergency services were called to the Norman Shaw Building, which hosts some parliamentary offices, "at 12.58 p.m.to reports of a suspicious package at Norman Shaw Buildings, Victoria Embankment, Westminster", a police statement said.
"We take religious hatred extremely seriously and my officers will be carrying out a full and thorough investigation in response to these reports", said Martin Snowden, head of counterterrorism policing in North East England.
The substance has since been found to be non-hazardous, according to police. The letters are now under investigation by the police.
The package contained a "low-level noxious substance", while the letter is a form one which encourages everything from verbal attacks on Muslims in the street to mosque bombings.
"Sadly, it is reflective of hate against Muslims which continues to manifest itself alongside the rise of the far-right", said the council, which represents some 2.7 million British Muslims.
City A.M. understands all staff at Parliament have been emailed warning them to be on alert.