The packet, which weighed around 150g, was found close to the seat of Leader of the Opposition Ram Govind Chaudhary, which is close to the podium where the Speaker sits.
"This is PETN... this is a dangerous substance," Chief Minister Yogi Adityanath told a stunned House soon after it assembled for the day.
"As informed by experts, 500 gm of this explosive is enough to blow up the House," he said.
PETN is one of the most dangerous plastic explosives which is available in black market, and belongs to the same family as nitroglycerin. PETN can be kept hidden in a sealed container or an electrical equipment and thus can easily surpass security checks.
Several countries have severe restrictions on purchase of PETN which can be bought in powder form or thin plastic sheet. The substance is legally used by the military and in mining industries where it is used in detonators for detonating cords and mines.
PETN can be mixed with other chemicals to form Semtex. Experts say PETN does not go off on its own. The explosive needs a secondary detonating mechanism to produce heat or a shockwave, which can detonate the explosive. The substance is stable and safe to handle, but requires a primary explosive to detonate it. PETN substance has been used in number of bombing incidents over the years.
Apart from reports of PETN explosives from all around the world, the substance is believed to have been used in the 2011 Delhi High Court blast, in which at least 17 people lost their lives.