(Reuters) - Egypt´s ruling military has approved a law banning Hosni Mubarak´s top former officials from running for president, ruling out his last prime minister and possibly helping former Arab League chief Amr Moussa in his race against Islamists.

Ahmed Shafiq´s elimination narrows the options for Egyptians 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.

Egypt holds its first presidential election since Mubarak´s overthrow in February 2011 on May 23 and 24, with a likely run-off between the top two candidates in June. The ruling generals are due to hand power to the new president on July 1.

The sketchy opinion polls that are available put Moussa in the lead, suggesting he will head 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 the Egypt´s presidency had already taken a dramatic turn earlier this month when the authorities disqualified three other front-runners including Mubarak´s former vice-president, Omar Suleiman, and two top Islamists.

The disqualifications have stoked the turbulence of the army-led transition, which has been punctuated by outbursts of lethal street violence and which is now mired in political tension between Islamists and secular-minded forces.

Egypt is set for more political uncertainty after the presidential vote. Questions remain about how much influence the powerful military will wield and to what extent presidential powers will be diluted in a new constitution.

"CONSTITUTIONAL TRANSGRESSION"

The law barring Mubarak-era officials was drafted by the Islamist- dominated parliament in response to former spy chief 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.

"In our view, any attempt to exclude or deprive citizen Ahmed Shafiq of his political rights is a constitutional transgression," said Ahmed Sarhan, spokesman for Shafiq.

Another spokesman for the Shafiq campaign said he would appeal against any move to disqualify him. The commission overseeing the election has the final say on Shafiq´s candidacy. Its decisions cannot be appealed in the courts.

Analysts say Shafiq´s exclusion seemed inevitable after the military council approved of the legislation, which took legal force on Tuesday, according to an official document.

The law denies political rights to anyone who occupied a string of top positions during the last decade of Mubarak´s rule, but does not apply to former ministers, meaning it does not affect Moussa´s bid. He served as foreign minister for a decade under Mubarak before becoming Arab League chief in 2001.

Mubarak appointed Shafiq as prime minister in his last days in power to try to defuse popular anger in the streets.

A former air force commander, 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; Writing by Tom Perry; Editing by Alistair Lyon) (© Thomson Reuters 2012. 04/24/12)