Tunisian President Moncef Marzouki on Friday asked Interior Minister Ali Larayedh, a hardliner from the main Islamist Ennahda party, to form a government within two weeks, his spokesman said.
Marzouki's spokesman told a news conference Ennahda leader Rached Ghannouchi had formally nominated Larayedh to succeed outgoing Prime Minister Hamadi Jebali, who resigned on Tuesday.
Ennahda is the biggest party in the National Constituent Assembly with 89 of its 217 seats. Marzouki's secular Congress for the Republic party, the second largest with 29 seats, has already said it will join a new Ennahda-led coalition.
But Ennahda's choice is likely to raise hackles among liberal Tunisians, some of whom accuse Larayedh's Interior Ministry of failing to curb violence by Islamists against advocates of secularism, including journalists and artists.
However, they give him credit for taking firm action against Islamist militants with alleged links to al Qaeda.
The assassination of secular opposition politician Chokri Belaid on February 6 plunged Tunisia into its worst crisis since the overthrow of strongman Zine al-Abidine Ben Ali two years ago.
Larayedh, 57, is viewed as part of Ennahda's hardline wing, which rejects any role for parties linked to the Ben Ali era.
A maritime engineer, Larayedh spent 15 years in jail under Ben Ali. He became interior minister when Jebali's government was formed in December 2011 after an election in October.
Jebali, who remains Ennahda's secretary-general, refused to head the next government after his own party rejected his plan for an apolitical technocrat cabinet to prepare for elections.
He had proposed the idea after Belaid's assassination, the first in Tunisia for decades. The killing ignited three days of mass protests that exposed deep rifts between Islamists and their opponents in a fledgling democracy with an ailing economy.
Political uncertainty has put negotiations on a $1.78 billion loan from the International Monetary Fund on hold and prompted Standard and Poor's to lower its long-term foreign and local currency sovereign credit rating on Tunisia on Tuesday.
(Writing by Alistair Lyon)