Four Muslim Labour MPs have been sent suspicious packages over the last 48 hours including copies of a “Punish a Muslim” letter that advocates violent acts against Muslims.

The Ealing Central and Acton MP, Rupa Huq, was the latest to receive the letter, copies of which are also understood to have been opened by staff for Rushanara Ali (Bethnal Green and Bow) and Mohammad Yasin (Bedford). A fourth package was sent to the Manchester Gorton MP, Afzal Khan, but was not opened.

A member of Huq’s staff has been taken to hospital as a precaution after opening the letter, which the MP said contained a “sticky substance” that the police had described as “low-level noxious”.

On Monday two people were taken to hospital, also as a precaution, when a package leaking a suspicious liquid was sent to Yasin’s office.

Police are investigating the “Punish a Muslim” letter, which has been distributed across the country and details a list of violent acts alongside numbers of points for performing them. Tell Mama UK, which monitors anti-Muslim hate crimes, said it had received reports of people in Bradford, Leicester, London, Cardiff and Sheffield getting the letters.

Huq said she had been assured that the substance was not harmful. “It is horrible though, it’s really very frightening,” she said. “It is to put the frighteners on us but we won’t be cowed. I’m sure I won’t be the last and I’m sure this is part of a pattern of targeting Muslim MPs. Among my colleagues, people have shown support but they are outraged.”

All four suspicious letters sent to MPs have been found in the Norman Shaw north building in Westminster, which houses MPs’ offices and is linked by a walkway to Portcullis House.

A parliamentary spokesman confirmed four packages had been sent to the building in recent days and said police were investigating the latest package.

“This is further to the two items investigated yesterday, both of which were found to be non-harmful,” the spokesman said. “The police are investigating the content of the letters to identify any potential link.”

A Metropolitan police spokesman said: “Police were called at 9.49am on Tuesday 13 March to reports of a suspicious package at Norman Shaw Buildings, Victoria Embankment, Westminster. Specialist officers have been called and the package will be assessed.”

The spokesman said there were no reported injuries and inquiries were continuing. Three police officers and five paramedics attended after Yasin’s staff reported the earlier package, and a man and a woman were taken to hospital as a precaution.

“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.”

Downing Street said it was concerned by the reports. “The prime minister is clear there is no space for racism, intimidation or extremism in the country or in parliament, so of course the reports are concerning,” Theresa May’s spokesman said. “The police are investigating this and it’s right we let them do so.”