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.
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 demanding her release.
Zaghari-Ratcliffe works for the Thomson Reuters Foundation, a charitable organisation coordinating training programmes for journalists around the world.
"She has nothing to do with Iran in her work and the foundation doesn't work with Iran anyway, so we have no idea why she has been detained for more than two months, first in isolation and then in a common cell," foundation CEO Monique Villa said last week.