Police are investigating after suspect packages containing anti-Muslim letters and a sticky liquid were sent to four Labour MPs over the past 24 hours.
Their Westminster offices were sealed off and three people were taken to hospital as a precaution. The substance was later found to be harmless.
The four MPs who received the letters are from South Asian backgrounds.
One of the MPs, Ealing Central and Acton's Rupa Huq, said her package was a "Punish a Muslim" letter.
The three other MPs to receive suspicious items were Bedford MP Mohammad Yasin, Bethnal Green's Rushanara Ali and Afzal Khan, MP for Manchester Gorton.
On Tuesday, a Parliamentary spokesman said items of mail containing suspicious substances had been investigated by the Met Police and were not found to be hazardous.
"The affected areas were temporarily cordoned off but no evacuation was necessary," the spokesman added. "One person attended hospital as a precaution."
Parliament has emailed all staff, warning them to "be on the alert for suspicious items of mail".
Ms Huq tweeted: "Staffer who went to hospital now discharged. There is a definite pattern. It was the letter that did the rounds at weekend and subject of Parliamentary urgent question yesterday which was received. Normal service now resumed - though scary experience."
She later told the BBC it was worrying the packages had got through security in the House of Commons but "it was even more worrying that someone out there thinks it is open season on Muslim MPs".
Labour leader Jeremy Corbyn said the letters and packages being circulated were "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.