The aviation enthusiasts deny the charges
Bank worker Antoni Adamiak, 36, who lives in London, spoke of his fear during a telephone call to his sister Maria Kennedy, 43.
He also complained that the prison he was being held in was "filthy" and "overcrowded".
The sole woman, grandmother Leslie Coppin, 51, was separated from the men, including her husband Paul Coppin, and taken to Korydalos women's prison in a suburb of Athens.
Mr Adamiak told his sister: "I am so frightened. The situation is getting desperate now."
He said his morale had "plummeted" and that he was "very frightened".
She said: "When he got to his cell he found his bed was a filthy mattress lying on the floor, about a quarter of the thickness of a normal one.
"He is sharing the room with three Greek men. I don't think they speak any English but they have been quite civil to him.
"He was alright in Kalamata because the group were all together and the camaraderie was keeping their spirits up. They were getting snippets of information and thought they might get out quickly.
Low morale
"But I think his morale has completely plummeted now. He doesn't know what will happen and whether it will take four days or four months to sort out.
"Antoni has asked me to do everything we can to get him out and I will, because we both feel the situation is getting desperate."
He was unsure where the others were moved to, although he knew some were placed in other wings of the same jail, she added.
Mrs Kennedy, who lives in Buckinghamshire, said she was worried about the reasons for the group being moved.
"Now they are in a proper jail, does that mean they intend to keep them for ages?" she said.
"They were in transit to the new place for seven hours and all they got to eat all day was some potatoes and onions in the evening."
She added: "I know plane-spotting has a bit of a bad name but my brother is not a nerd, he has a lot of interests. To say he is a spy is ridiculous.
"It is not a crime to be interested in aircraft, not something you should be incarcerated for in dire conditions for, especially not in a European country."
Divorced Mr Adamiak, who has a young daughter, said the British Embassy in Athens had been very helpful, bringing the detainees fruit and newspapers.
Court appearance
Consular staff were due to visit him on Sunday, after Greek authorities gave them special permission for a weekend visit, which is normally forbidden.
The group were on a plane-spotting tour when they were arrested in Kalamata last week and accused of taking photographs of a military airbase.
The plane spotters are due in court on Monday.
Others detained include Peter Norris, 52, of Uxbridge, west London, Wayne Groves, 38, of Tamworth, Andrew Jenkins, 32, from York, Mick Keane and Steve Rush, the Foreign Office confirmed.
Two Dutch men are also being questioned.