The bomb left on a London Tube train at Parsons Green contained 400g of an explosive that with just one gram could cause "serious injury", a court heard.
The device partially detonated on a District Line train on 15 September 2017, injuring many of the 93 passengers on board.
Prosecutors say Ahmed Hassan built the bomb and set the timer, before leaving it on the train one stop earlier.
He denies attempted murder and causing an explosion likely to endanger life.
Explosives expert Sarah Wilson said, had the device fully detonated, it had the potential to be "lethal".
'Unstable and unpredictable'
Ms Wilson was the first to give evidence at the Old Bailey on Friday.
She told the court that tests on the remains of the device showed the explosive chemical compound TATP had been used.
The jury heard how just one gram of the chemical compound could cause "serious injury to people or property", but that the device had contained around 400g, along with 2.2kg of shrapnel to cause further damage.
Ms Wilson described TATP as a "sensitive primary high explosive" which is "very unstable and unpredictable" – so is not produced for commercial or military use – and the bomb could be detonated just by being dropped.
It would have taken several hours to create that amount of the substance, she said, along with the small amount that was discovered at Mr Hassan's foster home in Sunbury, Surrey.
'Rapid burn'
Ms Wilson said all the elements necessary for making the bomb were there, but there could have been "a number of factors" around why it did not work properly, including whether it was made correctly or if it had become wet.
She said: "My conclusion was there had been a partial explosion. Obviously this device had not fully detonated but something quite violent happened.
"It was likely to be a rapid burn rather than a detonation.
"The initiator mechanism, the battery, has functioned but the main charge had failed to function fully."
House search
Another witness, counter terrorism officer Det Con Matt Rogers, told the court his team discovered the word "bored" written repeatedly on the door of Mr Hassan's bedroom when they searched his home on the day after the attack.
Officers found a bottle of hydrogen peroxide on his wardrobe, which prosecutors allege he used to make the bomb, and parts of a soldering kit were found underneath his bed.
They also discovered a ruler and supermarket loyalty card with traces of TATP on it behind a sofa in the house.
'Trained to kill'
On Thursday, the court heard from a number of witnesses to the explosion who described a "rolling fireball" coming towards them.
The jury also heard about injuries to some of the passengers, including hair being set on fire and clothes melting, after the device partially detonated.
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On Wednesday, the prosecution also said that Mr Hassan had told Home Office officials, 20 months before the Parsons Green attack, that he had been "trained to kill".
Prosecutor Alison Morgan said he was interviewed in Croydon in January 2016, where he is alleged to have told officials he had been taken by force by the so-called Islamic State group, who had threatened to kill his uncle and brother if he resisted.
The trial continues.
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