Cairo's Criminal Court said the convicted Al-Jazeera journalists “deliberately chose to publish false news to support the Muslim Brotherhood”.
The court released on Sunday a standard document setting out the reasoning behind their sentencing. The defendants can now appeal.
On 29 August, the Egyptian court sentenced six defendants, three Al-Jazeera journalists and three students, to three years in prison each.
The case garnered international and domestic condemnation.
Several countries have expressed their concern about the ruling, saying it could affect the future of freedom of expression and journalism in Egypt.
They were convicted of using unlicensed broadcasting equipment to publish false news on Al-Jazeera TV stations, and of participation in an outlawed group – a reference to the Muslim Brotherhood.
They were also convicted of obstructing governmental institutions and the law, attacking civil liberties and damaging national unity and social peace.
The reasoning document
In the document explaining their ruling the court said that Al-Jazeera's Mubasher TV channel lacked balance and objectivity in its coverage during and after the ouster of Islamist President Mohamed Morsi.
The court also said Qatar was using the Mubasher channel to publish news aimed at "bringing down the Egyptian state.”
The court said Egyptian-Canadian journalist Mohamed Fahmy and Egyptian journalist Mohamed Baher “set up a media center in the Marriot Hotel which is an unlicensed broadcasting space outside the areas designated by the state”.
The court said the three students - Sohaib Saad, Khaled Mohamed and Shadi Abdel Hamid – were members of the banned Muslim Brotherhood.
The lawyers of those convicted in the case have the right to appeal.
The three journalists were arrested in December 2013. Their trial and subsequent retrial have run for the past year and half.
Australian journalist Peter Greste, who was among those convicted, was deported to Australia in February 2015 under a 2014 presidential decree that allows foreign nationals to continue their pretrial detention or post-trial prison sentences in their home countries. He was sentenced in absentia.