Hawzah News Agency (London, UK) - The Ealing Central and Acton MP, Rupa Huq, was the latest to receive the letter over the last 48 hours after MPs Rushanara Ali and Mohammad Yasin.
In addition, the Manchester Gorton MP, Afzal Khan, received a fourth package which was not opened.
One of Huq’s staff members has been taken to hospital as a precaution after opening the letter, which apparently contained a "sticky substance" described by police as "low-level noxious."
Also on Monday, two people were taken to hospital as a precaution after Yasin's office received a package leaking a suspicious liquid.
"The substance contained in it was deemed not to be hazardous to health," a Met police spokesman said on Monday. "The incident, which was contained to one room, has been stood down."
According to a parliamentary spokesman, who confirmed that four packages had been sent to the building in recent days, police were currently investigating the latest package.
"This is further to the two items investigated yesterday, both of which were found to be non-harmful," said the spokesman. "The police are investigating the content of the letters to identify any potential link."
This comes after West Yorkshire Police said on Saturday that it had received around six reports of letters advertising "Punish a Muslim Day."
Londoners and Birmingham citizens also said they had received identical posts.
The letters pledged “rewards” for those who do violent acts against Muslims ranging from verbal abuse of them to burning or bombing mosques.
Downing Street has expressed concern about the reports, with Theresa May's spokesman saying that "the prime minister is clear that there is no space for racism, intimidation or extremism in the country or in parliament, so of course the reports are concerning."
London's Metropolitan police recorded more than 1,200 hate crime incidents against Muslim victims from April 2016 to March 2017, pointing to a surge of Islamophobia in the UK.