The fate of 150,000 US troops in Iraq is due to be decided today as the Iraqi Parliament gathers to vote on whether to approve an agreement with the United States that would allow US forces to stay in the country for another three years.

Failure to pass the pact, the product of nine months of negotiations between Baghdad and Washington, would mean the US military loses the authority to remain on the ground beyond the end of December and would have to return home.

It would also deal a blow to Nouri al-Maliki, the Prime Minister, and his Cabinet, which supports the agreement.

Hoshyar Zebari, Iraq’s Foreign Minister, described the vote as historic.

“It will be a crucial day, in my view, in Iraq’s history,” Mr Zebari told The Times. “We have to keep our fingers crossed.”

Ending a week of debates, lobbying and noisy protests, the 275-member Parliament is expected to vote on the pact by a show of hands.

A simple majority of 138 is enough to pass the text, but the Prime Minister will be looking for a clearer margin to give the agreement more legitimacy in the eyes of sceptics.

The status of forces agreement sets out a timetable for the phased withdrawal of US troops from Iraq over the next three years, starting with a pullout from all towns and cities by the middle of 2009.

It also puts US troops under the authority of the Iraqi Government for the first time since the 2003 invasion, replacing a United Nations Security Council mandate that is due to expire at the end of the year.

In addition, the text gives Iraq limited judicial rights over US soldiers and civilian Pentagon employees in the case of serious crimes off-duty and off-base.

Factions for and against the pact and those who are undecided held huddled meetings long into the night as MPs struggled to reach a consensus.

Shia Arab and Kurdish politicians in the ruling coalition are in favour of the security agreement, but support from the 44-seat Accordance Front, the Sunni Arab bloc, is needed to secure a comfortable majority.

The Accordance Front has said it will only give the nod if the public is allowed to vote on the deal in a referendum next year.

“The national division over the agreement is very clear,” said Tariq al-Hashemi, the Vice President and also a leading Sunni Arab MP.

“Consensus appears to be very difficult, if not impossible.”

Offering a more optimistic view, Ridha Jwad Teqi, a Shia Arab MP in the ruling coalition, said progress was being made on resolving political differences.

“The atmosphere of the negotiations was good,” he said, noting that those parties with objections “showed some flexibility”.

Grand Ayatollah Ali al-Sistani, Iraq’s most influential Shia cleric, has indicated he would not object to the agreement if passed by a comfortable majority. His support, tacit or otherwise, is vital to secure success.

Among the ‘No’ Camp, 30 Shia Arab MPs loyal to Moqtada al-Sadr, the anti-US cleric, remain fiercely opposed to the deal. Militant supporters of Hojatoleslam al-Sadr have warned of a return to armed resistance if it is passed.

Two hard-line Iranian newspapers also urged the Iraqi Parliament today to reject the pact. Iran enjoys a strong degree of influence in neighbouring Iraq.

Britain is waiting for the passage of the US-Iraqi agreement through Parliament before it seals a bilateral accord with Baghdad to enable British troops to remain in Iraq in 2009. Time is fast running out for the UK deal to be approved.

Times Recommends

• It’s only Brits who don’t appreciate me, says Blair
• Party’s over for Greek graspers
• White House tightens security for Xmas marathon