Nazanin Zaghari-Ratcliffe's prison term was disclosed Friday by her employer, the Thomson Reuters Foundation.
"This is a very serious condemnation that comes without any charges or evidence being made public," foundation CEO Monique Villa said in a statement.
Zaghari-Ratcliffe, who has dual citizenship, was arrested April 3. She's being held in Kerman prison in southeast Iran.
She had been visiting family in Iran with her 22-month-old daughter when she was detained at the airport in Tehran as she checked in to fly back to the UK. Her daughter was placed in the care of Zaghari-Ratcliffe's parents in Iran.
The Iranian government accused her of working for a UK media network allegedly involved in activities against the Islamic Republic, according to an IRGC statement released in June.
"By being a member of foreign companies and institutions, she has been participating in their scheme and implementation of their media and cyber projects for their soft overthrow of the sacred system of Islamic Republic," the IRGC said.
"She headed one of the main obstinate networks that have continued their criminal activities during the past few years, under the guidance and support of media and foreign government spy services. "
But Villa reiterated Friday that Zaghari-Ratcliffe had no dealings with Iran in her role as a project coordinator with the foundation, which is the charitable arm of the international news outlet Thomson Reuters. Villa said the foundation "does not operate in Iran directly or indirectly."
"I have instructed the Thomson Reuters Foundation's lawyers to find out what these charges are and I know that Nazanin's family has asked the same of their lawyer in Iran," Villa said. "As Nazanin's husband Richard says, this is a 'punishment without a crime' which is contrary to the rule of law."
Zaghari-Ratcliffe had been unable to visit her daughter or access a lawyer since being taken to prison, Richard Ratcliffe said in June.
The foundation said Zaghari-Ratcliffe had worked with the organization for the last four years. Ratcliffe said his wife trained journalists in developing countries such as Morocco and Burma, though she's never worked in Iran for the Reuters foundation.
Ratcliffe has launched a petition calling on the British Prime Minister and Iran's Supreme Leader Ali Khamenei to help free his wife.
Arrested at the airport
Zaghari-Ratcliffe moved to Britain in 2007, and has visited her family in Iran four times since having baby Gabriella almost two years ago, according to Ratcliffe.
The last time Ratcliffe spoke to his wife was over the phone on April 2. He fears his wife is in solitary confinement.
Their child, who has British citizenship only, had her passport confiscated by authorities, and remains with her grandparents in Tehran, according to Ratcliffe.
The International Campaign for Human Rights in Iran said the country does not recognize dual citizenship and that it "routinely harasses citizens and dual nationals with trumped up national security charges."
This means Zaghari-Ratcliffe is only recognized as Iranian, and as such is prevented from accessing British consular assistance.
The rights group told CNN that it was aware of several dual nationals imprisoned in Iran
. These include Iranian-Americans Siamak Namazi,
a businessman who has been held since October last year, and his 80-year-old father, Baquer Namazi,
who was arrested in February.