The Muslim Brotherhood in Egypt has nominated its deputy chairman, Khairat al-Shatir, as its candidate for the presidential elections in May.
The choice of Mr Shatir, the group's financier, reverses a pledge made earlier by the group's leaders not to contest the election.
It will raise concerns among liberals and the military that the Brotherhood could become too powerful.
Correspondents say its ties with the ruling council have steadily worsened.
There had been much speculation about whether it would opt to field a candidate following the party's legislative election success in November.
The movement's political arm then won around a third of the vote, and nearly half the seats in the first parliamentary election since the fall of Hosni Mubarak last year.
Mahmoud Hussein, the group's deputy leader, said it had decided to field a candidate following "attempts to abort the revolution". Only a few days remain before the close of nominations.
The announcement ends months of speculation about who the Muslim Brotherhood would throw its weight behind, says the BBC's Yolande Knell in Cairo.
Mr Shatir, a wealthy businessman, has long been a senior member of the Islamist group and its main financier.
He spent 12 years in prison because of his connection with the Brotherhood, which was previously banned. He was released only after last year's uprising.
In an official statement, the Muslim Brotherhood said it had reversed its decision not to contest the presidency to overcome risks to Egypt's revolution and the transfer from military to civilian rule.
The Brotherhood already dominates Egypt's newly elected parliament and the panel set up to draft the new constitution.
But it complains that its attempts to form a new cabinet have been blocked and there have been threats to dissolve parliament.
The ruling military council has been in conflict with the Brotherhood over the appointment of cabinet ministers.
The Brotherhood argues presidential candidates from the ousted government could present a further challenge.