Ealing Central and Acton MP Rupa Huq and Manchester Gorton MP Afzhal Khan were both sent suspicious items on Tuesday.
Ms Huq revealed one of her staff members was sent to hospital after handling an “irritant substance” but was later discharged.
Meanwhile, Mr Khan’s parliamentary office was evacuated due to an unopened suspicious package that was later found to pose no risk.
Both MPs spoke of a “pattern” of Islamophobic behaviour behind the incidents.
It comes after two people were sent to hospital as a precaution on Monday as police assessed a package sent to the office of their Labour colleague Mohammad Yasin, MP for Bedford and Kempston.
A fourth Labour politician, Bethnal Green and Bow MP Rushanara Ali, revealed she had opened a “threatening letter” on Monday calling for attacks on Muslims.
A parliamentary spokesman said: “The police are investigating the content of the letters to identify any potential link.”
Ms Huq posted an image on Twitter of the letter sent to her, which outlines a points-based rewards system for Islamophobic attacks as part of a “Punish a Muslim Day” on 3 April.
Counter-terror officers are currently investigating reports of similar letters in Yorkshire, with social media users also having reported receiving the letters in Birmingham and London.
MPs debated the wider reports of the letters in Parliament on Monday.
Ms Huq branded it “totally unacceptable” for Muslim MPs to receive such correspondence, while Mr Khan, although stating he had not received one of the racist pamphlets, said: “Clearly this is part of a pattern of Islamophobic behaviour with the ‘Punish a Muslim Day’ letters being sent to members of the public and to my fellow Muslim MPs”.
Commenting on the reports of hate letters, Theresa May’s spokesman said: “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.
“The police are investigating this and it’s right we let them do so.”
Labour leader Jeremy Corbyn described the “disgusting hate-filled letters” as “utterly contemptible”.
“Labour is committed to tackling Islamophobia and anti-Muslim hate crime, as well as all other forms of bigotry, racism and xenophobia,” he said in a statement.
“Those that seek to divide us will find that our country is united in mutual respect, proud of our diversity and ready to protect our shared future.
“Today we stand together with those facing this vile Islamophobia, to show them support, care and solidarity”.
Confirming the incident in Parliament on Tuesday, the Metropolitan Police 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 are on scene and the package is being assessed.
“The London Ambulance Service are on scene. A man has been taken to hospital as a precaution.
“No evacuation has taken place. A second package at the same location is also being assessed.”