The Hudood Ordinance law requires a woman who claims she's been raped to produce four witnesses.
Religious political parties have criticised an amendment to the law - which would have dropped the four-witness requirement - as un-Islamic.
But late on Monday night, S.M. Zafar, a prominent ruling-party lawmaker, said the government had agreed to a "compromise" by letting rape victims choose between prosecuting suspects under the Islamic four-witness rule, or under Pakistan's civil penal code.
He said: "It's a compromise which doesn't make [a] difference in the substance [of the law], but provides two different procedures for prosecuting a rape case."
Rape would remain punishable by death.
Politicians will receive a new draft of the proposed amendment bill on Tuesday.
Wasi Zafar, Pakistan's law minister, said: "If a woman has four witnesses she can file a case under the Hudood law, or if she does not have witnesses she can file a case under the penal code."
But Hafiz Hussain Ahmed, an opposition lawmaker, called it a victory for Pakistan's coalition of six religious parties.
He said: "Now they have acknowledged that the amendment was in conflict with the Quran."
The bill had originally been scheduled for legislative debate on Monday.
It was postponed until Wednesday after a panel of Islamic clerics, asked by the government for their opinion, suggested the adjustments.
The ruling Muslim League party has a parliamentary majority and could easily pass the bill.
Human rights groups have long been demanding that the Hudood Ordinance be entirely repealed.
The ordinance was approved by Zia ul-Haq, a former military dictator, in 1979.
Last month, the government presented a bill to amend the law, proposing that rape cases be tried only in civil courts, with no four-witness requirement.
But more than 60 lawmakers from the coalition of religious parties threatened to vacate their legislative seats, which could have forced a by-election and a major political crisis for Pervez Musharraf, the president.