Mr Hashemi, a Sunni Arab who along with the president and second vice-president has a veto over the Baghdad parliament, said he wanted more seats allocated to the millions of Iraqis - many of them Sunnis - living abroad.

Iraqis were already waiting for a date to be set for the vote after the law was held up by weeks of wrangling in the national parliament.

A compromise was eventually reached last week between MPs representing the country’s complex patchwork of religious and ethnic groupings, but only by leaving the most contested issues to be decided after the vote on a "wait-and-see" basis.

But Mr Hashemi has now referred the article back to parliament, leading the election commission to say it had put off preparations for the vote. That in turn threatens to delay the departure of American troops from Iraq, which was to be co-ordinated with further political progress in the country.

"The situation is very difficult, and it is clear now that this will affect the commission’s procedures and for sure will push the elections back," said Qassim al-Aboudi, a member of the commission.

Mr Hashemi wanted more seats to be allocated to minorities such as Christians and to Iraqis living in exile or as refugees, particularly since the latter are mainly also Sunni. Under current plans, the two groups have a guaranteed five per cent of the 323 seats, and Mr Hashemi wants that figure increased to 15 per cent.

His decision to veto the law was bitterly criticised by the prime minister, Nouri al-Maliki, who is a Shia. He called it "a serious threat to the political process and democracy".