President Pervez Musharraf, who has few allies left in the assembly after their defeat in an election on February 18, asked the assembly to meet on March 24, spokesman Rashid Qureshi said.
"The president has been pleased to summon the National Assembly to ascertain the member who commands the confidence of the majority of members," Qureshi said.
Musharraf, who came to power as a general in a coup in 1999, appears increasingly isolated, and there is intense speculation over how long the US ally will be able to hold onto power.
The incoming government has pledged to pass a resolution to reinstate Supreme Court judges replaced by Musharraf in November out of fear they could rule his own re-election by the previous assembly as unconstitutional.
The Pakistan People's Party (PPP) of assassinated former Prime Minister Benazir Bhutto will lead the coalition, but it has still to name its candidate for the premiership.
It could be a stop-gap prime minister, if Bhutto's widower and political successor Asif Ali Zardari ultimately decides he wants the job himself.
Bhutto's 19-year-old son Bilawal Zardari Bhutto has returned to Pakistan and is expected to reveal the party's choice on Monday.
"We have time, so we can announce it either on Monday morning or just before the session," said PPP spokesman Farhatullah Babar.
The February election saw the PPP emerging with the most seats in the 342-member National Assembly, but not enough to rule alone.
Former Prime Minister Nawaz Sharif's party came second, dealing a crushing defeat to the pro-Musharraf Pakistan Muslim League.
Sharif, the leader Musharraf deposed, and Zardari signed an agreement this month to form a coalition with a small regional party and a religious party.
Neither Zardari nor Sharif stood in the election.
Makhdoom Amin Fahim, a senior aide to Bhutto and Zardari's deputy, had been favourite but his prospects dimmed after Sharif's party objected to his contacts with Musharraf.
There have been growing calls from within the PPP for Zardari to take up the job, but for now he is not eligible as he is not a National Assembly member.
However, he could win a by-election to take his place in the assembly should he want to become prime minister. (Reuters)
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