Anoushka Marashlian, an independent Middle East analyst, told Al Jazeera that the exact charges against Rassam are unclear.
"He's been seen as acting against the national interest, but they haven't clarified as to what he has been charged with," she said.
"I think [the British government] needs to wait to see what the actual charges are. Is he being charged with treason, which is the accusation that some have levelled against supporters of Mousavi?"
The announcement that a trial was likely came during Friday prayers in Iran.
By then, eight British embassy workers had been arrested, though seven have since been released.
Ayatollah Ahmad Jannati, a prominent figure in Iran's clerical leadership, said embassy staff had allegedly stoked post-election unrest and that some would be put on trial.
Jannati does not hold a position in the government, but is the head of the Guardian Council, a powerful body in the clerical hierarchy that stands above the elected government.
Iran's action is likely to increase its isolation and alienate Western nations that have been trying to keep options open with Tehran despite its crackdown on protesters.
The British government has said it is seeking "urgent clarification" from Tehran over the charges.
David Miliband, the British foreign secretary, had earlier said the Iranian embassy staff had done nothing wrong to warrant their arrests on Friday.
Britain is pressing for members of the European Union to pull their ambassadors out of Tehran to protest against the staffers' arrests.
Maziar Bahari, a Newsweek journalist, has also been accused of acting against national security.
Iran's Fars news agency reported an "interview" with Bahari, in which he said that he had filed "unreal and biased reports from Iran which were driven by greed".
Newsweek repeated a call on Iran to release its correspondent immediately and rejected charges made against him on Wednesday.
The US weekly magazine said that Bahari has been detained in Iran since June 21 without access to a lawyer.
Saleh Nikbakht, the lawyer representing Bahari and several other detained reformist leaders being accused with the same charge, said: "I still have not been able to meet him [Bahrai] despite going to the prosecutor's office several times."
Nikbakht also said that most of his clients have not even been able to contact their families.
"Any kind of interview and confession by these people who are being held in prison is invalid under the law and the Iranian constitution," Nikbakht said.
Newsweek quoted an Iranian state news agency as saying that Bahari "has said he participated in a Western media effort to promote irresponsible reporting in Iran".
In addition to Bahari, a Canadian-Iranian national, Iranian authorities have also expelled Jon Leyne, a BBC correspondent, and Jason Fowden, a Greek-British journalist for the Washington Times.