Abd al-Aziz al-Hakim, head of the Supreme Council for the Islamic Revolution in Iraq (SCIRI) which is represented on the US-appointed Iraqi Governing Council, said Sayyid Ali al-Sistani, widely revered as Iraq's most influential Islamic leader, had expressed misgivings about the plan.
Al-Sistani rarely makes public pronouncements on politics but his status means that any criticism of the timetable for the transfer of power could mean many Shias reject the plan.
"He expressed concern about real gaps, which must be dealt with or the plan will lack the ability to meet the hopes of the Iraqi people. It diminishes the role of the Iraqi people in the process of transferring authority to Iraqis," al-Hakim told a news conference in the holy city of Najaf.
"He didn't find anything that assures Islamic identity...there should have been a stipulation which prevents legislating anything that contradicts Islam in the new Iraq, in
either the interim or permanent phase."
Shia Muslims make up 60% of Iraq's population and their leaders have largely sidestepped conflict with the US administration, in the hope of playing a leading political role in Iraq's next government.
According to the new plans unveiled earlier this month by Washington and the Governing Council, regional caucuses will select a national assembly by the end of May, and this will pick a transitional government by the end of June.
The government would take over sovereignty from occupying powers in July, and a constitution would be written and democratic elections held by the end of 2005.
Al-Hakim said Iraq's tense security situation was not an obstacle to a popular vote to elect the national assembly, instead of selection by regional caucuses.
"It would be possible to hold elections in 80% of Iraq," he said, adding that in Sistani's view the Governing Council should have pursued original plans to form a constitutional committee in December.
"The new plans could have "consequences that will lead to more suffering for Iraqis"
Abd al-Aziz al-Hakim,
Head of the Supreme Council for the Islamic Revolution in Iraq
"He (al-Sistani) didn't see any reason for the delay," al-Hakim said, also faulting the new timetable for sidelining the United Nations.
The new plans could have "consequences that will lead to more suffering for Iraqis," al-Hakim said, declining to elaborate.
SCIRI, which opposed Saddam Hussein's government for years from exile in Iran, has cooperated with Iraq's US-led administration, drawing criticism from some Iraqi Shias.
Al-Sistani has been largely silent on the occupation, but another Shia leader, Muqtada al-Sadr, has repeatedly condemned the US presence in Iraq.