A main ally of Pakistani Prime Minister Imran Khan's government has reached a pact with opposition parties seeking to oust him, a party official said on Wednesday, signalling Khan may be running out of options as he tries to stay on in power.

Former cricket star Khan, 69, is battling a series of defections from his ruling alliance and an increasingly united opposition trying to force him from office in a no-confidence vote due on Monday.

The Muttahida Qaumi Movement (MQM), Khan's largest ally in the National Assembly, said it has reached an agreement with the main opposition bloc, and would announce details later in the day.

"An agreement between the united opposition and MQM has taken final shape," MQM Senator Faisal Subzwari said on Twitter, adding party officials of the two sides were reviewing the agreement.

Khan's party does not have a simple majority in parliament by itself and needs the support of coalition allies.

Opposition parties accuse Khan of mismanaging the economy, foreign policy and resorting to heavy-handed measures against critics.