The official, who asked not to be named, said the Movement for Democratic Change no longer had confidence in the negotiations mediated by South African President Thabo Mbeki.
"He (Mbeki) is trying to rush us into a deal. The unfortunate thing is that Mbeki is trying to help Mugabe achieve his ends, not to solve the crisis."
State media reported on Thursday Mugabe is ready to go ahead and form a cabinet if opposition leader Morgan Tsvangirai does not sign a power-sharing deal on Thursday.
The MDC said any attempts by Mugabe to pressure it into an agreement would fail.
"Where on earth have you seen dialogue held on the basis of threats and ultimatum? They want to bully us into an agreement, but that’s completely unacceptable," MDC spokesman Nelson Chamisa told Reuters.
Zimbabwean state media have described talks on Thursday as the last chance for Tsvangirai to agree to a deal to end post-election political deadlock that has worsened Zimbabwe’s economic decline.
But analysts believe neither side has any option but to agree a deal eventually.
So far talks have been blocked over how executive power should be shared. Tsvangirai has rejected a proposal he says gives Mugabe control of Zimbabwe’s powerful security forces.
Mugabe, in power since 1980, said he would form a government with or without Tsvangirai because Zimbabwe could not afford to have a situation where "we will not have a cabinet forever", the state-run Herald newspaper reported.
"If after tomorrow (Thursday) Tsvangirai does not want to sign we will certainly put together a cabinet. We feel frozen at the moment," Mugabe told reporters in Lusaka on Wednesday after attending the funeral of Zambian President Levy Mwanawasa.
Tsvangirai beat Mugabe in a March 29 election but fell short of enough votes to avoid a June run-off vote, which was won by Mugabe unopposed after Tsvangirai pulled out citing violence and intimidation against his supporters.
The election was condemned around the world and drew toughened sanctions from Western countries whose support is vital for reviving Zimbabwe’s ruined economy.
Zimbabwe state radio said Mbeki was expected to arrive in Zimbabwe on Thursday to continue mediation efforts. Mbeki’s spokesman said there was no truth to the reports. Reuters