"I am returning the trust reposed in me by the nation and the National Assembly today," Chaudhry Shujaat Husayn told the assembly on Wednesday.
"Mr Shaukat Aziz will be elected as prime minister within a few days."
Husayn sent his resignation to President Pervez Musharraf who accepted it, but had asked him to continue as prime minister until his successor assumed the office, a senior government official said.
Vote of confidence
The National Assembly would elect the new prime minister on Friday and he would be sworn in and face a vote of confidence, which is also a formality, on Saturday, said Shaikh Rashid Ahmad, information minister in the outgoing cabinet.
A senior government official, who declined to be identified, said some changes were expected in the cabinet expected to be announced by Aziz on Saturday and it was likely to be larger than the outgoing one. He gave no details.
Husayn took office on 30 June, replacing Zafarullah Khan Jamali, who had a falling out with President Musharraf. Husayn was keeping the seat warm for Aziz, who needed to win a National Assembly seat before taking up the post.
Aziz, a former Citibank executive credited with turning around Pakistan's economic fortunes, won two seats in by-elections last week, one of which he will retain.