By Frances Harrison
BBC News, Tehran
A former top Iranian nuclear negotiator arrested on Monday is to be charged with spying, Iran's conservative Fars news agency has suggested.
Mr Mousavian was a top nuclear negotiator in talks with Europe
It quotes an unnamed source saying that Hossein Mousavian is to be charged with espionage relating to nuclear affairs.
Tehran's public prosecutor has confirmed that Mr Mousavian is being held in prison.
Mr Mousavian, an ex-envoy to Germany, was a member of Iran's negotiating team with Europe in the last government.
Fars quotes the source saying that Mr Mousavian is to be charged in a Revolutionary court.
Earlier, the Tehran public prosecutor told the Iranian media that the charges against Mr Mousavian would only be announced once the investigation was finished and the final verdict given.
He said the intelligence ministry was dealing with the case.
One website close to the president, Raja News, has quoted an informed source alleging Mr Mousavian was spying for Germany.
What has shocked many Iranians is that Mr Mousavian is an establishment figure, not a reformist or critic of the Islamic system, and yet he is now behind bars.
Because Mr Mousavian is close to former President Hashemi Rafsanjani, some have suggested the arrest could also be connected to a power struggle within the establishment.