By Shaimaa Fayed and Tom Perry
CAIRO (Reuters) - Hosni Mubarak's last prime minister was thrown out of Egypt's presidential election on Tuesday because of ties to the former administration, a disqualification that could help former Arab League chief Amr Moussa in a race against his Islamist rivals.
Ahmed Shafiq's elimination all but guarantees that Egypt will be led by a president who does not hail from a military background for the first time since army officers overthrew the king in 1952.
The ex-air force commander was seen by some as the preferred choice of the ruling army generals when he first decided to run.
He was ruled out by the commission overseeing the vote after the military rulers approved a law drawn up by the Islamist-dominated parliament to strip political rights from top Mubarak-era officials. Moussa had served as Mubarak's foreign minister but the law does not apply to officials at ministerial level.
Shafiq's removal narrows the options for voters who do not want an Islamist head of state. The other front-runners are the Muslim Brotherhood's Mohamed Mursi and Abdel Moneim Abol Fotouh, a former member of the group who said he was confident of victory in an interview with Reuters on Tuesday.
The few opinion polls that are available have put former Arab League chief Moussa in the lead, suggesting he will make it into the run-off against one of the Islamists. However, many voters are undecided.
"Part of the votes that would have gone to Shafiq would go to Amr Moussa, because he is the only figure with government experience that remains in the race," said Mustapha Kamel al-Sayyid, a professor of political science at Cairo University.
The first real race for Egypt's presidency had already taken one dramatic turn when earlier this month the authorities disqualified three other front-runners, including Mubarak's former vice-president, Omar Suleiman, and two top Islamists.
The army-led transition has been overshadowed by bursts of street violence and rising tensions between Islamists and secular-minded politicians at odds over the new constitution.
Egypt holds its first presidential election since Mubarak was ousted in February 2011 on May 23 and 24, with a run-off between the top two candidates likely in June. The ruling generals who assumed power from Mubarak last year are due to hand power to the new president on July 1.
Political uncertainty is unlikely to end with the presidential vote. Questions remain about how much influence the powerful military will wield after the vote and to what extent presidential powers will be diluted in the new constitution.
Abol Fotouh, a member of the Brotherhood for decades, moved back to the heart of the race when his rivals, including the group's first-choice candidate, were disqualified.
The Brotherhood expelled Abol Fotouh last year when he defied its wishes by deciding to run for president. On Tuesday, he told Reuters he expected to win outright by securing more than 50 percent of the vote in the first round. "God willing, we will take most of the Brotherhood's votes," he said.
"We are working and organizing on the basis that we will win from the first round and not in the run-off," the 60-year old said in an interview. "As elections approach, our chances of winning are increasing."
A diplomat in Cairo said the race still appeared to be very open. "The recent developments tend to favor the centrist candidates: Moussa and Abol Fotouh," said the diplomat, speaking on condition of anonymity.
The law barring Mubarak-era officials from running was drafted by parliament in response to former spy chief Omar Suleiman's late decision to enter the race. News of his candidacy brought tens of thousands of demonstrators - both Islamists and liberals - into the streets in protest.
Ahmed Sarhan, a spokesman for Shafiq, said the ban ran against the constitution. "In our view, any attempt to exclude or deprive citizen Ahmed Shafiq of his political rights is a constitutional transgression".
The election commission's decisions cannot be appealed in the courts.
Mubarak appointed Shafiq as prime minister in his last days in power to try to defuse popular anger in the streets. Shafiq had served as Mubarak's minister of civil aviation and won credit for overseeing the redevelopment of Cairo airport.
(Additional reporting by Marwa Awad, Dina Zayed and Yasmine Saleh; Writing by Tom Perry; Editing by Andrew Osborn)