"Morsi was held in conditions that can only be described as brutal, particularly during his five-year detention in the Tora prison complex," a statement said.
His death "after enduring those conditions could amount to a state-sanctioned arbitrary killing", the experts added.
Egypt's first democratically elected civilian president Morsi died in June after collapsing in a Cairo courtroom while on trial.
President Abdel Fattah-al Sisi led the military ouster of the Islamist leader in 2013 when he was head of the army.
Morsi, who belonged to the now outlawed Muslim Brotherhood, was overthrown after a tumultuous year in power.
He spent nearly six years in solitary confinement where the panel said he "was denied life-saving and ongoing care for his diabetes and high blood pressure".
The experts noted Friday that "authorities were warned repeatedly" about his deteriorating health to the "point of killing him".
"There is no evidence they (authorities) acted to address these concerns, even though the consequences were foreseeable," the statement added.
Egyptian authorities have not yet commented on the panel's findings.
Led by special rapporteur Agnes Callamard and the UN Working Group on Arbitrary Detention, they explicitly called out Sisi's government for human rights violations in jails describing them as "intentional".
Since ascending to the presidency in 2014, Sisi's government has waged a wide-scale crackdown jailing thousands of Islamists, secular activists, and even bloggers and comedians.
The independent panel of UN experts also warned that thousands of prisoners are faced with severe health risks due to medical negligence in jail.
They listed documented reports of overcrowding, inadequate food, poor ventilation, no access to sunlight, denial of family visits as well as failure to receive medical care.
Calling Morsi's death "unlawful", the panel recommended publishing an impartial investigation of all prisoners who have died in state custody since 2012.
Amr Darrag, former international cooperation minister under Morsi and a senior Brotherhood official, welcomed the experts' findings.
"This investigation is a significant step forward in holding such regimes accountable for their actions," he said.
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