The seven -- five men and two women -- were sentenced to 20 years in prison in August on charges that include acting against Iran's national security, collaboration with foreign countries, and "corruption on earth."
Their sentences were later halved, but the campaign group Amnesty International and the Baha'i International Community said that Iranian authorities have reversed that decision.
Diane Ala'i, the community's representative to the United Nations in Geneva, told RFE/RL's Radio Farda it appeared the chief prosecutor had asked for an annulment on the grounds that the reduced sentences were not in line with Shari'a law.
Ala'i said the lawyers for the seven would appeal against what she described as an "illegal" decision.
She said the jailed Baha'is, their lawyers, the Baha'i International Community, as well the international community were united in calling for justice.
In Washington, State Department spokesman Mark Toner said the United States remained concerned with the "continued persecution of Baha'is and other religious minority communities in Iran."
"We're deeply troubled about reports coming out of Iran that a 20-year sentence of the seven Baha'i leaders was reinstated on appeal by the prosecutor-general, a man the United States recently imposed economic sanctions and a travel ban on for committing serious human rights violations," Toner said.
"We condemn this unprecedented step as a violation of Iran's obligations under the International Covenant on Civil and Political Rights."
In a statement, Amnesty International condemned what it called a "vindictive" and "outrageous" decision and made a renewed call for the Baha'is' release.
The Baha'i faith is not recognized under the Iranian Constitution, and its estimated 300,000 followers are subjected to systematic harassment and persecution.