"Now we have no exact day of election. It's a very difficult situation," he told Al Jazeera.
The latest vote setback will worry Washington, which is already concerned about the threat of violence around the election.
The US state department expressed disappointment and urged Iraqi leaders to move quickly to resolve the situation.
"We are disappointed at these developments related to [the] elections law," spokesman Ian Kelly said.
"We urge Iraqi leaders and parliament to take quick action to [resolve] any outstanding concerns that have been expressed and this is so elections can go forward."
General Ray Odierno, the US commander in Iraq, warned of more trouble in the run-up to the vote and said he could yet ask Washington to alter the plan to have all US combat troops out of Iraq by next August, ahead of a complete military exit in 2011.
"We believe that there will be an attempt to conduct more attacks between now and the election," he said, noting that he has up until April or May next year to decide "if we have to defer from the August 31 date that the president has set".
Ban Ki-moon, the UN secretary general, also called for parliament to move swiftly to pass the necessary legislation so that "the election can
Al Jazeera's Mosab Jasim, reporting from Iraq, said al-Hashemi's move "means we are back to square one".
After weeks of wrangling, MPs finally passed the electoral law on November 8 for the second national election since Saddam Hussein was ousted from power in 2003.
Al-Hashemi says he wants 'to make the representation of Iraqis abroad fairer'
But under the constitution, any member of the presidential council can veto a proposed law up to two times before the bill is returned to parliament for approval by a vote of at least 60 per cent.
The presidential council, comprised of Jalal Talabani, the president who is a Kurd, and two vice-presidents, one Sunni Arab and one Shia, has demanded a greater say in the election for nationals living abroad and minorities.
Al-Hashemi, the Sunni Arab vice-president, said "the modification aims to make the representation of Iraqis abroad fairer".
"It's not only a matter of those displaced to neighbouring countries but of all Iraqis of all confessions and religions who live abroad."
The Sunni Arab community, which was dominant under Saddam's rule, puts the number of Iraqis living abroad at around four million.
Talabani had said on Monday that he wanted parliament to revise the election law so that the number of seats set aside for minorities and Iraqi expatriates is tripled, from five to 15 per cent – the same level that governed Iraq's first post-Saddam general election in December 2005.
|Masoud Barzani wants more parliamentary seats for the Kurdish region [AFP]
Kurdish political leaders also expressed concerns about the election law on Tuesday and threatened to boycott the polls unless Kurdish areas received more seats in parliament.
The office of Masoud Barzani, the Kurdistan regional president, said the way seats were distributed after the passage of the law was unfair to Kurds.
"Unless this seat allocation formula is reconsidered in a just manner, the people of [the] Kurdistan region will be compelled to boycott the election," a statement posted on Barzani's website said
Parliament must now reopen debate on the bill and the electoral commission believes there is too little time to complete preparations by the scheduled mid-January polling date.
Al-Maliki called on parliament to convene urgently to tackle the amendments to allow elections to go ahead as planned in January.
He also urged Iraqis to support their MPs and thwart plans by the country's "enemies" to take advantage of a vacuum if the elections were delayed.