The explosive was found by the cleaning staff.
The assembly has unanimously recommended a probe by the National Investigation Team (NIA) into the incident.
Pentaerythritol Trinitrate is one of the most risky and powerful explosives in the world.
Sources said, the explosive was found under the seat Samajwadi Party MLA Manoj Pandey.
The CM seeks an NIA probe in the incident.
The suspicious white powder named as PETN or Pentaerythritol tetranitrate was seized on July 12 as a result of which Chief Minister Yogi Adityanath has called an emergency meeting at 10.30 am on Friday. The powder was then sent for analysis at a forensic lab which identified it as PECTIN - an extremely powerful explosive that is hard to detect.More news: Leonardo Bonucci on verge of shock AC Milan transfer
Yesterday, in the Uttar Pradesh Assembly he said: "Everyone needs to be cautious related to security matters". It is being viewed as a major security lapse as before the session begins, a dog squad is pressed into service along with security men to check inside the Vidhan Bhawan when the session is on. The powder was sent to the laboratory for testing, which confirmed that it was PETN.
He appealed to Speaker Hriday Narain Dixit to ensure police verification of the assembly staff because security of over 500 MLAs and MLCs was at stake. On all the gates QRT of Provincial Armed Constabulary (PAC) will be deployed and whole body scanners will be installed.
PETN is relatively stable and is detonated either by heat or a shockwave.
PETN was also used in the bomb plot on a Northwest Airlines Flight 253 in which a passenger tried to set off plastic explosives sewn to his underwear in 2009.
Yogi has also assured protection to the devotees, who participate in thousands in the one-month religious event, Saavan.