Senior judge Mr Justice Jeremy Baker stopped proceedings in court two of the Old Bailey today after hearing that Hashem Abedi, 22, was feeling "worse not better".
Adjourning until tomorrow, he told jurors: "I have arranged for more inquiries to be made about his condition so as to ensure if he needs any medical intervention it is provided properly and effectively.
"It is not appropriate we continue in the absence of Mr Abedi at this time so what I'm going to do is say we will adjourn for today so that these matters can be resolved."
On May 22 2017, Salman Abedi, 22, carried a homemade bomb in a rucksack into the foyer outside the 21,000 capacity arena and detonated it as concert-goers were leaving an Ariana Grande gig, killing 22 and injuring many more.
Among those killed were Sorrell Leczkowski, 14, from Adel, Leeds; Courtney Boyle, 19, a Leeds Beckett University student from Gateshead; Kelly Brewster, 32, from Sheffield; Wendy Fawell, 50, from Otley; and Angelika and Marcin Klis, a couple from York.
The prosecution allege Hashem Abedi is "just as guilty" as his older brother because they say he helped him stockpile chemicals and materials for making the bomb.
Earlier, jurors and families of those killed in the blast sitting in court were told of Hashem Abedi's alleged attempts to source the ingredients for explosive TATP of a kind his brother Salman Abedi used to construct the devastating device.
In early March 2017, five litres of sulphuric acid - one of three components of TATP - was bought on Amazon by a young man on behalf of Hashem Abedi, the court heard.
Around that time, he allegedly turned to a friend for help to get more, saying he needed it for a generator battery in Libya to replace some that had been spilled by his brother, jurors heard.
He told his friend, who cannot be identified for legal reasons, that he was "skint" so could not buy it himself, prosecutor Duncan Penny QC said.
An order for £76 worth of acid on Amazon was declined due to a lack of funds, prompting the friend to speak to his father, the court heard.
But the father refused to help saying that the chemical could be used to manufacture explosives, it was alleged.
Mr Penny said: "He ignored further calls which were made to him by the defendant over the coming days."
Days later, Hashem Abedi was in contact with a youth who searched the internet for "sulfuric acid" at a cost of £69.36, jurors were told.
Two days later, another Amazon account, belonging to Hashem's friend Mohammed Younis Soliman, was used to order 10 litres of sulphuric acid, jurors heard.
Mr Penny said £140 in cash was later paid into Mr Soliman's account.
The prosecutor, examining phone records of various associates, told jurors that Sim cards were regularly "swapped" between smartphones and those not connected to the internet, sometimes staying in a handset for just a few moments.
He added: "There is a direct connection between this defendant and the first order of hydrogen peroxide."
The defendant, originally from Manchester, denies the murder of 22 men, women and children aged between eight and 51.
He also denies attempted murder and conspiring with his brother to cause explosions.