Iraq's outgoing Prime Minister Iyad Allawi says his party will join the interim government of his successor, Shia leader Ibrahim Jaafari.
Allawi oversaw Iraq's first post-Saddam elections
Mr Allawi took the decision "because he believes in making the political and democratic process in Iraq successful," spokesman Thaer al-Naqib said.
The prime minister's bloc came third in the January election, winning 40 seats in the 275-member parliament.
The secular Shia leader had previously said he wanted to remain in opposition.
But correspondents say the inclusion of the Iraqi National Accord in the Iraqi government may help assuage Sunni fears about a cabinet dominated by Islamist Shia Muslims.
Ibrahim Jaafari was appointed on Thursday shortly after Kurdish leader Jalal Talabani was sworn in as Iraq's new interim president.
The transitional government's main task will be to oversee the drafting of a permanent Iraqi constitution and to pave the way for elections in December.
Mr Jaafari, 58, has up to a month to name his team, but has indicated that he hopes to announce a new government within two weeks.
Mr Allawi is said to be negotiating which posts his colleagues will take in the new cabinet.
The outgoing prime minister is not expected to be among them.