Pakistan’s two main political dynasties reached an agreement late on Tuesday to form a coalition government, ensuring that candidates aligned with former Prime Minister Imran Khan will not take power despite having won the most seats in last week’s election.

Leaders of the party favored by the country’s powerful military, the Pakistan Muslim League-Nawaz, or P.M.L.N., announced that they had joined forces with another major party, the Pakistan People’s Party, and others to reach a two-thirds majority in the incoming Parliament.

“This is not a time for disagreements, but to unite,” said Shehbaz Sharif, a former prime minister whom the coalition said it would nominate to regain that post. “Let’s move forward, move the economy forward, end mutual differences.”

The announcement came five days after a national election that most had expected P.M.L.N. to win easily after it received the backing of the military, which has frequently engineered electoral outcomes. In a stunning upset, candidates allied with Mr. Khan routed the two longstanding major parties, defying a monthslong military crackdown on their own party and delivering a sharp rebuke to the country’s generals.

The election results set off days of political jockeying. As P.M.L.N. and the P.P.P. discussed joining forces, the country waited to see if Mr. Khan, who is serving multiple jail sentences on charges he says are politically motivated, could pull off another upset and form a coalition of his own.

While Tuesday’s announcement confirms that members of Mr. Khan’s party, Pakistan Tehreek-e-Insaf, or P.T.I., will sit in the opposition in Parliament, they will be far from sidelined, potentially posing a serious challenge to the coalition government.

P.T.I. has accused the military of tampering with the vote count in dozens of races and has promised a long, bruising court battle to challenge the results. For many Pakistanis, those accusations damage the legitimacy of Mr. Sharif’s coalition, which P.T.I. leaders have accused of forming a government with “stolen votes.”

The P.M.L.N. announcement on Tuesday that the coalition would nominate Mr. Sharif to serve as prime minister ended days of speculation over whether he or his older brother, Nawaz Sharif, a three-time prime minister, would lead the next government.

The younger Mr. Sharif, 72, led a coalition government after Mr. Khan’s ouster by Parliament in 2022. He is seen as more deferential to the military than is his brother, who has fallen out with the country’s generals multiple times after pushing for more civilian control of the government.

Putting forward the younger Mr. Sharif as prime minister suggests that the military will retain its tight grip on the country’s politics from behind the scenes, analysts say.

The previous coalition government led by Mr. Sharif was deeply unpopular and criticized for failing to address an economic crisis that has sent inflation to record highs in recent years. The country’s economy has been surviving on a $3 billion bailout from the International Monetary Fund.

“It will be an uphill task for Shehbaz Sharif’s government,” said Muneeb Farooq, a Lahore-based political analyst. “The expected economic turnaround which everyone keeps talking about is far from reality.”

P.M.L.N. leaders said that the elder Mr. Sharif’s daughter, Maryam Nawaz Sharif, will be nominated to lead the provincial government in Punjab Province, the home of the Sharif political dynasty and where it has recently faced a strong challenge from P.T.I.

The move was seen as an effort by the elder Mr. Sharif to pass on the party’s baton to his daughter.

Asif Ali Zardari, a co-chairman of the Pakistan People’s Party, and officials from several other smaller parties expressed support for Shehbaz Sharif to form the next government after the leaders held a meeting in Islamabad, the capital, late on Tuesday night.

“We will take Pakistan out of difficult times,” said Mr. Zardari, who is expected to be the country’s next president, a largely ceremonial post. His wife, Benazir Bhutto, was a two-time prime minister of Pakistan and was assassinated in 2007.