Nazanin Zaghari-Ratcliffe, an employee of the Thomson Reuters Foundation, was accused of being "involved in the soft overthrow of the Islamic republic through... her membership in foreign companies and institutions," Iran's elite Revolutionary Guards said, quoted by the Mizan news agency.
Zaghari-Ratcliffe, 37, was arrested at Tehran airport on April 3 as she prepared to return to Britain with her daughter after visiting family in Iran, her husband Richard Ratcliffe said last week.
Iran does not recognise dual citizenship and, if put on trial, she will be considered an Iranian.
According to a Guards statement, Zaghari-Ratcliffe was "identified and arrested after massive intelligence operations" as one of "the heads of foreign-linked hostile networks".
She was alleged to have conducted "various missions... leading her criminal activities under the direction of media and intelligence services of foreign governments".
"Further investigations are being done and her case has been sent to Tehran for legal proceedings," the statement added.
Zaghari-Ratcliffe is being held in a furnished room in a prison in the southeastern city of Kerman, it added.
Her husband, who last spoke to his wife on May 30 and has said she was held in solitary confinement for 45 days, on Wednesday scoffed at the charges levelled against her.
Raised at 'highest levels'
"It's complete nonsense. It's taken them 70 days to come up with this, and it's still not clear what it means anyway," he told AFP.
"Her father has appointed a lawyer. The next step is to discuss with her father what the lawyer knows. And I'll probably go to the Iranian embassy," he added.
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Their two-year-old daughter, whose British passport was taken away, has stayed in Iran under the care of her grandparents.
Britain's Foreign Office said it has raised the case of Zaghari-Ratcliffe "repeatedly and at the highest levels" and will continue to do so at "every available opportunity".
"We have also been supporting Mrs Zaghari-Ratcliffe's family since we were first made aware of her arrest. Minister for the Middle East, Tobias Ellwood, has met personally with the family to reassure them that we will continue to do all we can on this case."
The husband, who said Zaghari-Ratcliffe was initially told a "passport problem" was the cause of her being detained at Tehran airport, organised a rally outside the Iranian embassy in London on Friday calling for her release.
Her employer, the charitable arm of financial information and news giant Thomson Reuters, coordinates training programmes for journalists around the world.
Chief executive Monique Villa said Wednesday that Zaghari-Ratcliffe had travelled to Iran on a family holiday.
"Nazanin has been working at the Thomson Reuters Foundation for the past four years as a project coordinator in charge of grants applications and training, and had no dealing with Iran in her professional capacity," she said.
"The Thomson Reuters Foundation has no dealings with Iran whatsoever, does not operate and does not plan to operate in the country.
"We remain in close contact with Nazanin's family and her husband Richard, as well as with the British Foreign Office. We are assessing the situation as it evolves and hope the matter will be resolved as soon as possible."
In 2012, an Iranian court found the Reuters news agency guilty of "propaganda against the regime", and "publishing false information in an effort to disturb public opinion" in a report portraying female ninja students as assassins.
Iran withdrew the press credentials of all staff at Reuters' Tehran bureau over the story and suspended the work of the news agency, part of the New York-based Thomson Reuters group.
Reuters was given permission to reopen the bureau the following year but it has yet to do so.