JUI-F chief Maulana Fazlur Rehman announced the decision while speaking to the media alongside opposition leaders following an important meeting between the two sides.
He said it was the first meeting with the opposition and that the parties had agreed in principle to establish a united opposition, while further consultations would continue.
Sources said a joint parliamentary party meeting of the opposition parties would also be held tomorrow, after which the opposition alliance would meet Maulana Fazlur Rehman.
The meeting also discussed the health condition of Pakistan Tehreek-e-Insaf (PTI) founder Imran Khan, according to sources.
Asked about the possible participation of the Pakistan Peoples Party (PPP), Maulana Fazlur Rehman said the PPP was part of the government, while the current discussions concerned opposition parties. He added that the proposal for an all-parties conference should also be considered.
When asked whether the opposition parties would join hands only in Parliament or also take their campaign to the streets, the JUI-F chief said further decisions would be taken after consultations.
The development comes as Prime Minister Shehbaz Sharif has once again invited the opposition to hold negotiations, saying political issues should be resolved through dialogue to promote economic development and strengthen democracy.
The Prime Minister, while chairing a federal cabinet meeting in Islamabad, said the government had consistently expressed its readiness for constructive dialogue and that it was now up to the opposition to decide when it was prepared to come to the negotiating table.
Meanwhile, the government has highlighted progress on the economic front, with the Prime Minister noting an 18 percent increase in IT exports in July. He stressed the need to expand digital technology across all sectors and fully digitize the governance system.
Shehbaz Sharif also appreciated Finance Minister Muhammad Aurangzeb and his team for introducing a fixed tax scheme for retailers, directing the Federal Board of Revenue (FBR) to ensure that retailers are not subjected to harassment.
He also credited the petroleum minister and his team for the reduction of diesel prices by Rs32 per litre, describing the move as a welcome development.
Referring to tensions in the Gulf region and the Strait of Hormuz, the Prime Minister said Pakistan was continuing diplomatic efforts to reduce tensions and improve the regional situation, stressing that stability in the region was important for Pakistan's economy.