A judge is set to rule on the plane-spotters case
They will face fresh espionage charges after magistrates in Greece examined intelligence service reports on photographs and notebooks belonging to them, their defence lawyer told the BBC.
The 12 Britons are already accused of taking photographs of a military airbase.
BBC correspondent Paul Wood, speaking from Greece, said: "What started out as complete joke is now a very serious incident."
The group were detained on 8 November in the town of Kalamata, while on a plane-spotting holiday.
The 12 all deny taking photographs inside a restricted military zone, a charge which carries a maximum 20-year jail sentence in Greece.
Lawyer Yannis Zacharias said: "It seems that the experts from the air force, in their report, produced findings that not only support the existing charges, but have led the investigating judge to prepare to bring further charges against these people."
A new charge would be trespass at an airfield which no civilian could approach, he said.
They would be brought back before the judge either at the end of this week or next Monday, he said.
Mr Zacharias said:"I believe the majority of these people had nothing to do with the incidents.
He had said on Monday he believed the "most likely" outcome would be that the charges would be reduced.
Eleven British and two Dutch men are being held in a prison in Nafplion, about 80 miles from Athens.
The sole woman, Briton Lesley Copping, 51, is in the Korydallos high security prison in the capital's suburbs - the only one in the area with a women's wing.
The group's lawyers have had problems in court explaining plane-spotting, which Mr Zacharias said was "not a well-known hobby in Greece".
'Good spirits'
British consul Donald Holder, who visited the tourists on Saturday, said they were all in "remarkably good spirits", apart from Mrs Coppin who was separated from the other members of the group when they were transferred from the police holding cells in Kalamata.
Mrs Coppin's husband Paul, 45, from Mildenhall, Suffolk, is the boss of Touchdown Tours, the company responsible for arranging the week-long trip.
Another of the Britons held in Greece was allowed to see his father on Tuesday.
Wayne Groves from Tamworth, was visited at the prison by his father Don.
The 12 were initially accused of taking photographs of a military air base, but the charges were increased to full-blown espionage after the discovery of notebooks allegedly containing details of two other airfields, including a Nato base at Araxos in southern Greece.
Greek police named the other 10 Britons as: Peter Norris, 52; Antoni Adamiak, 37; Steven Rush, 38; Andrew Jenkins, 32; Christopher Wilson, 46; Wayne Groves, 38; Graham Arnold, 38; Michael Keane, 57; Gary Fagan, 28; Michael Bursell, 47.