The US Justice Department says Iraqi national Mohammad Baqer Saad Dawood Al-Saadi attempted to pass coded messages to two other alleged Iran-backed operatives through phone calls to immediate family members from jail in New York.
Al-Saadi had previously pleaded not guilty to eight criminal counts including conspiracy to provide material support to Kata'ib Hezbollah, an Iran-backed militia the US considers a terrorist organisation.
Five men were arrested outside RAF Fairford airbase in September over the security incident, with two others arrested later on suspicion of assisting Iranian intelligence.
The incident came amid heightened concern about the security implications for Western states of the war between the US and Iran.
RAF Fairford had been used by US bombers to carry out strikes on Iran.
British Prime Minister Andy Burnham said in October the activity at the airbase was likely to have been co-ordinated in Iran.
While Tehran's ambassador to Britain has denied such accusations, Iran's Revolutionary Guards said in July that the Fairford base and any other base used to launch attacks against Iran were legitimate targets
A lawyer for Al-Saadi declined to comment.
In a court filing on October 14, Al-Saadi's lawyers said the prosecution's assertion that he was using coded language during phone calls with family was speculative and based on questionable translations from Arabic. They said there were banal explanations for conversations the prosecution flagged as concerning.
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