The package was sent to Muslim Labour MP Mohammad Yasin. A fourth package was sent to Manchester Gorton MP Afzal Khan, but was not opened.
The letter was also sent to households nationwide, said Tell MAMA, a charity that investigates anti-Muslim acts in the UK.
A member of Huq's staff has been taken to hospital as a precaution after opening the letter, which the MP told the Guardian was a "sticky substance" and the police had described as "low-level noxious".
Another Muslim Labour lawmaker, Rushanara Ali, tweeted late Monday that she "had the extremely unpleasant experience of opening an envelope with [the] very same threatening letter". Huq, who said she was in a select committee meeting at the time of the incident, has been MP for Ealing Central and Acton since 2015. "It is to put the frighteners on us but we won't be cowed".
"Tell MAMA documented a year ago 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".
All four packages have been found in the Norman Shaw north building in Westminster, which houses MPs' offices and is linked by a walkway to Portcullis House.
'Specialist officers attended and the package was assessed. "The police are investigating the content of the letters to identify any potential link".
The London police announced on Monday that two policemen were "hospitalized" after discovering a suspicious package at the Norman Shaw Buildings, a pair of buildings in Westminster used as Parliamentary offices, reports AFP on Tuesday.
"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.
Police officers and paramedics rushed to the scene and two people have been taken to hospital as a precautionary measure.
A Commons spokesperson said: "Today a suspicious substance was investigated by the Met Police and was found not to be hazardous".
'The incident, which was contained to one room, is in the process of being stood down'.