MPs passed the law, which sets out the process for general elections scheduled for January, on November 8 after weeks of wrangling.
But al-Hashemi, a Sunni Arab, rejected the measure saying he objected to the first article in the law because it did not give a voice to Iraqis abroad, many of whom are Sunni Muslims who fled the country during sectarian fighting after the US-led invasion in 2003.
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.
Baha'a al-Aaraji, the head of the legal committee in the Iraqi parliament, said the country's supreme federal court had ruled on Thursday that veto was unconstitutional.
But this was disputed by a number of MPs, including Samarie, who said the court opinion had not addressed the legality of Hashemi's veto and any such opinion would not be legally binding anyway.
Saleh al-Mutlaq, an independent Sunni MP, said: "To my knowledge, the federal court did not say the veto is not constitutional."
"They are trying to create a real political crisis."
The veto has put the national elections, which must be held before January 31 under the constitution, in jeopardy with the electoral commission halting its work organising the polls until the law is finalised.
Nouri al-Maliki, Iraq's prime minister, called the veto a "dangerous threat to the political process and democracy".
He hit out at al-Hashemi's actions and urged the commission to continue preparations "without any delay".
Washington has expressed concern over the latest setback and it urged Iraqi leaders to move quickly to resolve the situation.
"We are disappointed at these developments related to [the] elections law," Ian Kelly, a US state department spokesman, said.