Iraq's president on Monday asked Haider al-Abadi, the Shi'ite coalition's nominee for prime minister, to form a government, a spokesman for the main Shi'ite coalition said.
The move, which comes after months of political wrangling, is likely to be resisted by Nuri al-Maliki, who has rejected calls to drop his bid for a third term as prime minister.
Iraq's highest court earlier issued a ruling suggesting Maliki's State of Law Shi'ite bloc is the biggest in parliament and therefore was entitled to nominate a candidate for prime minister.
Maliki's aides were not immediately available for comment. He has defied calls by Sunnis, Kurds, some fellow Shi'ites and regional power broker Iran to step aside for a less polarising figure.
Critics say Maliki alienated Sunnis, prompting them to support Islamic State militants who have seized a large chunk of northern Iraq and have threatened to march on Baghdad, posing the biggest threat to Iraq since the fall of Saddam Hussein in 2003.
State television showed footage of the president shaking hands with Abadi and telling him: "I hope you will be successful in forming a broader-based government."
Abadi is a low-key figure who spent time in Britain. According to his Facebook biography, his favourite quotation is "the key to leadership is tolerance".
Sectarian violence has become widespread in Iraq again, reaching levels seen when a civil war peaked in 2006-2007.
Educated at the University of Manchester, Abadi served as the head of parliament's finance committee, a political advisor to the prime minister and minister of communications.
The Godfather and The Matrix are among his favourite movies, according to a Facebook entry.