The case of Pastor Yousef Nadarkhani has twice been referred to the Supreme Leader of Iran, who has yet to issue a decision.
Christian Solidarity Worldwide said that a final ruling is expected at anytime and that it had received unconfirmed reports indicating that execution may be delayed for up to a year to allow time to convince the pastor to renounce his faith.
The organisation said it had also received reports that another jailed pastor, Behnam Irani, was suffering as a result of "frequent" assaults by inmates.
He is serving a five year sentence in Ghezel Hesar prison in Karaj for carrying out an "action against national security".
The beatings have reportedly been so severe that he now has difficulty walking.
There are reports that the pastor was held incommunicado in a small cell where guards would repeatedly wake him from sleep as a form of psychological torture.
The verdict against Pastor Irani stated that he was an apostate and reiterated that apostates "can be killed".
Another Church of Iran member, Alireza Seyyedian, was sentenced to six years in prison in November for "propagating against an order of the regime and acting against national security".
His lawyers have yet to receive written confirmation of the verdict, which was issued verbally.
CSW's Advocacy Director Andrew Johnston said, "CSW is deeply concerned for the health and welfare of Pastor Irani, and urges the Iranian government to ensure that he, and all prisoners, are protected from assault, receive necessary medical attention, and are treated in accordance with the Standard Minimum Rules for the Treatment of Prisoners, as defined by the UN.
"It is also unacceptable that the Iranian regime continues to portray legitimate religious activities of minority faiths as a national security threat, and to subject people to unjust and lengthy prison terms on this basis.
"The international community must maintain pressure on Iran during the Christmas period and throughout next year to ensure that the rights of minority religious groups are respected, and to secure the acquittal and release of Pastor Nadarkhani and of all others unjustly imprisoned or facing execution due to flawed judicial processes."