Six stunned British plane-spotters were last night preparing to fly home from Greece with the shadow of a three-year jail sentence hanging over them.

Lawyers acting for the six, and two Dutchmen, launched an immediate appeal after they were convicted of spying at a military air show. The judge ruled that they would be allowed home pending that hearing.

Among them was 32-year-old Andrew Jenkins, from York. And last night his sister, Adele, of Bishopthorpe, York, reacted with shock and disbelief at the sentence.

She said: "I am just so upset, I can't talk about it. I don't even know what is happening."

City of York MP Hugh Bayley said: "The sentence is completely disproportionate and should be overturned on appeal. The Greek authorities may not understand or like plane spotting, but a prison sentence is wrong and a waste of Greek taxpayers' money."

Another six Britons were given one-year sentences yesterday by the court in the southern town of Kalamata after being convicted of aiding and abetting, but the sentences were immediately suspended.

Downing Street said Tony Blair was closely following the case and had previously raised it with Greek Prime Minister Costas Simitis.

The Prime Minister's official spokesman said: "The Government has always believed that the response to this case has been disproportionate and will continue to give the defendants and their families as much help as it can."

Amid chaotic scenes following the verdict delivered by presiding Judge Potoula Fotopoulou, both the plane-spotters and their relatives expressed their dismay.

The leader of the group, Paul Coppin, said: "I'm disgusted with the whole legal system. It's a shambles."

His wife, Lesley, said: "I cannot believe they have done it. This is all about politics. They made a mistake and they won't back down."

Mrs Coppin's son, Stephen Warren, said: "It's insane. They are a bunch of anoraks with a strange hobby, but it's not something they should be jailed for."

The 12 Britons are due to fly back to the UK today.

Defence lawyer Yannis Zacharias said the appeal could take up to two years and would probably be heard in Kalamata.

The group, who have spent £17,000 each in administrative expenses and legal costs since their arrest, were detained last November. The case sparked anger when the group were held in prison for almost six weeks before being allowed to return to Britain on £9,000 bail each.

They claimed they were pursuing an innocent hobby and gathering information readily available in books and on the Internet.

During the trial, the spotters stressed that a Brigadier General in the Greek Air Force had given them permission to be at the air base in Kalamata.

But prosecution witnesses told the court the spotters could have endangered Greek national security, while Squadron Leader Nektarios Samaras said the group's notes were too expert to have been made by amateurs.

Prosecutor Panatiotis Poulios told the court: "They all knew that it was classified information which could put the defence of the country in danger."

Euro MP Richard Howitt, who has campaigned for the plane-spotters since the start, said the verdicts were a "complete travesty".

Stephen Jakobi, director of Fair Trials Abroad, said: "I've forecast all along that if the Greeks got this one wrong, the shockwaves would be felt throughout Europe."