Australian journalist Peter Greste says his detention in an Egyptian jail is an attack on freedom of the press.

Australian journalist Peter Greste has described his solitary confinement in Egypt as an attack on freedom of speech.

He’s spent just four hours outside his cell since being arrested on December 29 on suspicion of collaborating with the blacklisted Muslim Brotherhood.

After the “glorious” relief of the weak winter sun, he wrote his first letter to his parents, which was smuggled out of the maximum security Tora Prison, south of Cairo.

Greste, 48, and two of his al-Jazeera colleagues were arrested in a hotel on suspicion of collaborating with the Brotherhood, hosting meetings, and broadcasting false information to further their aims.

While Greste is yet to be charged, he has been in 24-hour solitary confinement.

Producers Mohamed Fahmy Baher Mohameare are detained in the far more draconian “Scorpion prison” built for convicted terrorists.

On Thursday, Greste was told by the prosecutor general his initial 15-day detention would be continued for a third time to give investigators more time.

“He can do this indefinitely, one of my prison mates has been behind bars for six months without a single charge,” he wrote from his cold prison cell after his first official exercise session.

“I’ve been caught in the middle of a political struggle that is not my own.

“Our arrest and continued detention sends a clear and unequivocal message to all journalists covering Egypt, both foreign and local.

“The state will not tolerate hearing from the Muslim Brotherhood or any other critical voices.

“The prisons are overflowing with anyone who opposes or challenges the government.

“Anyone, in short, who refuses to applaud the institution.”

Greste, from Brisbane, works as East Africa correspondent for the global network and was due to be in Egypt for only a short time, relieving an absent colleague.

His father Juris is desperate and hopeful Greste will be released, and has chosen to stay in Australia to campaign rather than fly to Egypt.

“We hope it will happen at the end of this third detention period,” he told AAP.

“We have been informed, that at the moment for cases of this kind, his detention could be extended for up to two years.”