Cairo, Egypt (CNN) -- The political arm of Egypt´s Muslim Brotherhood has announced plans to run one of its leaders in the country´s presidential elections in May, reversing an earlier pledge to stay out of the race.

The once-banned Islamist movement will be represented by Khairat al- Shater, a longtime financial backer, the Brotherhood announced over the weekend. Al-Shater has resigned from his post as deputy chairman to join the already crowded field of presidential candidates, group said.

The jail terms he served under ex-Egyptian strongman Hosni Mubarak had been an obstacle that would have kept him off the ballot. But the Supreme Council of the Armed Forces, which took power after the 2011 uprising that toppled Mubarak, pardoned him Sunday, his lawyer, Abdel Moneim Abdel Maqsood, told CNN.

The Muslim Brotherhood has pledged repeatedly that it would not field a presidential candidate. But candidates from its political arm, the Freedom and Justice party, won the largest share of seats in Egypt´s parliamentary elections in December. And Brotherhood leader Mohamed Badie said Saturday the new Egypt "is under a serious threat" because its current, military-led government "has failed to represent the will of the people."

More than 450 people have already registered or announced plans to seek the presidency. Among them are former Arab League Secretary- General Amr Moussa -- who served as Mubarak´s foreign minister -- and Ayman Nour, an opposition leader jailed by Mubarak and recently pardoned as well.

The field also includes other Islamist presidential hopefuls, including the ultra-conservative Salafist candidate Hazem Abu Ismael and former Muslim Brotherhood member Abdel Moneim Abou El Fettouh, who broke with the Muslim Brotherhood over what he called its authoritarian style.

Al-Shater is a furniture and textile magnate who has led the Brotherhood´s business association. Though considered a conservative, he is also credited with being the driving force behind the Brotherhood´s affirmation that Egypt should continue to honor its international agreements -- including its peace treaty with Israel.

Liberals and secularists who led the uprising against Mubarak fear that a victory in the presidential elections, the first round of which begins May 23, may lead the Brotherhood to impose a fundamentalist Islamic agenda on Egypt.

"Their stance continues to change as they strengthen their political position," socialist activist Sherif Maher said. "They were patient after Mubarak fell and announced that they would not seek more than 20% of the seats in parliament. The number went up to 30, and now they have won more than half of the constituent assembly."

But Rami Shaath, a founding member of the Egyptian Revolutionary Alliance, said al-Shater´s entry into the race may be a bid to make an example of Aboul Fettouh, "who had defected against their will."

"Aboul Fettouh is championed by the revolutionaries and not favored by the military," Shaath said. "They also want to make a point to the youth of the Brotherhood that abandoning the group may cripple one´s ambition."

In March, the Muslim Brotherhood blasted the Supreme Council of the Armed Forces for holding onto power despite the parliamentary elections and questioning whether the generals would try to rig the presidential vote. In a rare and charged public response, the generals hit back against what they called a "baseless slander" and an "unacceptable" challenge to the legitimacy of elections.

Al-Shater is a 61-year-old civil engineer who became a millionaire businessman. He has been involved with Islamist groups since the late 1960s, according to his official biography, and was jailed for five years by a military court during a crackdown on Islamist movements in the mid-1990s.

In 2007, he was charged with providing funds and weapons to college students and imprisoned again. He was still behind bars when the regime fell in February 2011, and the military junta that took power from Mubarak released him for medical reasons a month later. Before Sunday´s pardon, that record could have disqualified him from the race. CNN´s Ben Wedeman contributed to this report. (© 2012 Cable News Network 04/02/12)