Jailed Australian journalist Peter Greste will launch a legal appeal against his conviction and seven-year jail term for defaming Egypt and aiding banned Islamists.

His family will detail plans for the appeal in Brisbane on Friday, as the journalist himself released a statement saying it will be a complex legal and political challenge.

Greste has begged supporters around the world to continue fighting for him and his Al Jazeera English colleagues, saying that as long as they remain behind bars, Egypt’s fledgling democracy “wears a muzzle”.

An Egyptian court last month handed Greste and his Canadian-Egyptian colleague Mohamed Fahmy seven-year jail terms for falsifying news and aiding the now blacklisted Muslim Brotherhood of former president Mohamed Morsi, who was ousted by the army last July.

Another colleague, Baher Mohamed, who is Egyptian, was jailed for 10 years. All three remain in Cairo’s Tora prison.

The Australian reporter’s youngest brother, Mike Greste, will detail plans for an appeal on Friday.

Meanwhile, Peter Greste and Baher Mohamed have told, in a statement, of their shock at last month’s verdicts.

They said they’d clung, perhaps naively, to the hope they would be cleared given the total lack of evidence presented at their trial.

When the guilty verdicts came, it was like a body blow, the men said.

“We heard nothing in the seconds that followed. Others have spoken of the uproar that erupted in the packed court, but all we heard was a stunned silence,” they said.

“Then came the numbness – a bizarre sense of unreality, as if we were watching it unfold on TV to a bunch of other poor souls. Our ears had heard the words; our brains had interpreted them, but our hearts refused to feel them.”

They said reality only began to dawn when they returned to their cell.

“There was a nausea, the gasping for air, the shaky knees, the oppressive weight of the concrete and iron cell weighing down on our shoulders with a heaviness we had never felt before. Seven to 10 years for crimes we did not commit.”

The men said the legal battle ahead would be extremely challenging, and urged supporters around the world to maintain pressure on Egypt to free them.

“As long as we remain behind bars, all of Egypt’s press works with the threat of imprisonment hanging over it, and the nation’s fledgling democracy wears a muzzle,” they wrote.