The group must pay about £9,000 each in bail
The lawyer - who was sitting in a hearing with the judges considering the case - told the BBC the courts were asking for bail of five million drachmas, about £9,100, for each defendant.
If supplied the Britons can go back to the UK but will have to return to Greece at a later date to face a trial on a lesser misdemeanour charge of illegal information collection, which carries a five-year sentence.
The same conditions also apply for the two Dutch plane-spotters arrested with the group.
The group's lawyer, Yannis Zacharias, said: "We are very pleased as this was our objective all the way through."
But Stephen Warren, the son of one of the accused Lesley Coppin, branded the decision "disgusting" and said there was little hope of raising the bail money in time for their release before Christmas.
Ron Arnold, the father of Graham Arnold, told the BBC that he hoped the government would help the families to raise the bail money.
Maria Kennedy, the sister of Antoni Adamiak, said: "I am just desperate to get him home. I just want to know now what I need to do practically and what I need to do about this money.
Victory
"I won't really believe it until he is back and I can see him."
Labour East MEP Richard Howitt, who has been campaigning on behalf of the group, said he did regard the decision as a victory.
"I have no doubt that when it comes to trial the case will collapse - although technically this misdemeanour allegation could carry a five-year jail sentence if proved.
"It does worry me that asking each family to find £10,000 for bail is a lot. We are now just hoping that these people will be back home as soon as possible."
Mr Zacharias said he was not sure when the accused would be released because it depended on when bail was paid.
The group were arrested on 8 November in the Greek town of Kalamata at after attending an airshow.
The state prosecutor had recommended in a 30-page report to the judges that they should be tried.
The 12 British people in custody are Paul Coppin, 45 and Lesley Coppin, 51, of Mildenhall, Suffolk; Peter Norris, 52, of Uxbridge, west London; Antoni Adamiak, 37, of London; Andrew Jenkins, 32, of York; Wayne Groves, 38, of Tamworth; Michael Bussell, 47, of Swanland, near Hull; Michael Keane, 57, of Dartford, Kent; Steven Rush, 38, from Caterham, Surrey; Christopher Wilson, 46, who lives close to Gatwick Airport; Graham Arnold, 38, from Ottershaw, Surrey; and Gary Fagan, 28, from Kegworth, Leicestershire.
The two Dutchmen being held are Patrick Dirksen, 27, from Eindhoven, and Frank Mink, 28, from Den Helder.