Human rights lawyer Mohammed al-Baqer and blogger Mohammed "Oxygen" Ibrahim were also jailed for four years on the same charge, Mona Seif tweeted.

They have been detained since September 2019, when the authorities cracked down on rare anti-government protests.

Human rights groups had called the charges spurious and the trial a sham.

Human rights groups have accused President Abdul Fattah al-Sisi's government of suppressing any form of dissent since he led the military's overthrow of his predecessor Mohammed Morsi in 2013. They say tens of thousands of government critics, including activists and human rights lawyers, have been detained arbitrarily.

Amnesty International said Monday's verdict was a "travesty of justice" while the Committee to Protect Journalists called it "unacceptable".

Alaa Abdel Fattah, 40, a software developer and blogger, first rose to prominence during the 2011 uprising that forced long-time President Hosni Mubarak to resign.

Two years later, after Mr Sisi came to power, he was arrested for organising a protest in defiance of a new law that banned unauthorised demonstrations. He was subsequently sentenced to five years in prison and was released on probation in March 2019.

That September, Abdel Fattah, Mohammed al-Baqer and Mohammed Ibrahim were among more than 2,000 people reportedly detained by security forces in response to protests against government corruption that included calls for the president's resignation.

The three men were accused of "spreading false news undermining national security" and "using social media to commit a publishing offence", as well as "joining a terrorist group". Human rights activists said the charges stemmed from their criticism of the crackdown and treatment of prisoners.

They spent more than two years in pre-trial detention - more than is allowed under Egyptian law - before being referred to the Emergency State Security Court for trial on the false news charges this October.

The move came just 10 days before the president lifted a nationwide state of emergency that allowed for such referrals. The emergency courts' verdicts are not subject to appeal before a higher court, but they require final approval by the president.

Human Rights Watch said on Sunday that the trial of the three men had been marred by numerous fair trial and due process violations.

Abdel Fattah's family told the group that his lawyers were only able to view the case file in the presence of prosecutors, and that the court consistently ignored their requests for a copy of the document.

The defendants also faced what Human Rights Watch described as "horrific" conditions at Cairo's Tora prison.

Abdel Fattah's family said he had been beaten and humiliated in detention, denied access to reading materials, the radio and the prison library, and not given recreational time outside of his cell.

The Arab Network for Human Rights Information said authorities had deprived Ibrahim of visits by his lawyers and family since February 2020, even after he attempted suicide this August.