Daniel James, 45, was arrested in 2006 when he was working for General David Richards, who was then commanding international forces in Afghanistan and is now head of the British army.
Reservist James, a salsa teacher in civilian life, was convicted by a jury at the Central Criminal Court, or Old Bailey, of sending coded emails to the Iranian military attache in Kabul.
Jurors will continue their deliberations Thursday on a second charge against him relating to a memory stick containing secret documents found in his possession plus a third count of misconduct in a public office.
James, who styled himself "General James", was born in Tehran and his trial heard from colleagues that he had expressed sympathy for Iran in conversations with them.
He also got within 20 metres (yards) of then British prime minister Tony Blair and took pictures of him when he visited Afghanistan in 2006. James told a colleague he did not like Blair, the court heard.
Prosecutor Mark Dennis accused James of "the height of betrayal" and the court was told that senior intelligence officers believed his acts could have cost the lives of British soldiers and possibly endangered Britain itself.
James denied the charges against him, saying he was trying to help set up a gas deal between Afghanistan and Iran which he hoped would promote trade and peace between Iran and the United States.
