Mr Greste, along with his colleagues and the Al-Jazeera network, have repeatedly denied the allegations levelled against them.

On Wednesday, the prosecutor's office said 16 Egyptians in the case are accused of joining a terrorist group, while the foreigners - an Australian, a Dutch citizen and two Britons - are accused of helping to promote false news benefiting the terrorist group, Associated Press reported.

Mr Grest is just one man swept up in a tidal wave of thousands of arrests in Egypt in the last seven months, and he says the message of the military-backed government to those trying to report the news is clear and menacing.

“Our arrest has served as a chilling warning to others of where the middle is here,” he writes in a second letter smuggled from his cell in Tora Prison in the capital Cairo.

“The state here seems to see itself in an existential struggle that pits the forces of good, open, free society against the Islamist 'terrorists' still struggling to seize control.

In that environment, ‘normal’ has shifted so far from the more widely accepted ‘middle’ that our work suddenly appeared to be threatening.”

Greste, an award-winning journalist for Al-Jazeera, along with his two colleagues, producers Mohamed Fahmy and Baher Mohamed, are accused of collaborating with the Muslim Brotherhood to use unlicensed equipment to broadcast information they knew to be false to defame and destabilise Egypt.

They were arrested on December 29 on what Greste describes as a very routine reporting assignment to produce a set of “relatively uncontroversial stories”.

Fahmy and Baher are further accused of being members of the Brotherhood – now designated a terrorist organisation in Egypt.

All strenuously deny the allegations, and to date the three have not been charged with any crime, and recently had their detention extended by another 15 days while Egyptian prosecutors continued to gather evidence.

“Under Egypt's judicial system, we won't get to see the file until charges are formally laid,” Greste writes.

“So, all we have is what we did – a routine body of reporting on the political drama unfolding around us, and what it might mean for Egypt. The fact that this has put us behind bars is especially alarming given the historical moment Egypt now finds itself in.”

Since the former Muslim Brotherhood-backed president Mohamed Mursi was deposed by the army on July 3 following a wave of protests thousands have been arrested – some estimates put that number at more than 21,000, in addition to the 1049 authorities say were arrested on January 25 alone.

Al-Jazeera is due to hold a press conference in London on Wednesday calling for the release of their news team, at which Peter Greste’s parents are expected to speak via satellite from Brisbane.