President Robert Mugabe and MDC leader Morgan Tsvangirai agreed to form a unity government in September, but the deal has stalled over disagreements on the control of key ministries.
The state-run Sunday Mail reported that the constitutional amendment bill – creating the office of prime minister for Tsvangirai – had been published on Saturday. The MDC immediately rejected the move, saying it had not been consulted.
"This was done unilaterally by (the ruling party) ZANU-PF," MDC spokesman Nelson Chamisa told Reuters.
"The gazetting was supposed to have been done after consultations."
He said the MDC had not seen the published Bill to establish whether it conforms with the draft agreed by the two parties during talks held in South Africa last month.
Concerns must be addressed first
Chamisa said the MDC wanted its concerns on the allocation of ministerial posts and provincial governorships addressed before the constitutional amendments could be dealt with.
"What we are saying is that these political issues will stand in the way of the legal process. We need to clear the political issues first before moving on to the constitution," Chamisa said.
On Saturday, state media quoted Justice Minister Patrick Chinamasa as saying Mugabe could call fresh elections if the opposition-dominated parliament fails to pass constitutional changes for the unity government.
Tsvangirai’s MDC won 100 seats in the 210-member lower house of parliament in a March poll as ZANU-PF lost its majority for the first time since 1980, garnering 99 seats. The balance is held by a smaller faction of the MDC, led by Arthur Mutambara.
Tsvangirai beat Mugabe in a presidential poll held concurrently, but fell short of the necessary votes to avoid a run-off poll which the 84-year-old veteran leader won after Tsvangirai pulled out of the race citing violence.
The second vote was widely condemned and Mugabe has come under renewed Western pressure to step down in the face of a cholera outbreak that has killed nearly 800 people, worsening the plight of Zimbabweans grappling with an economic meltdown blamed on government mismanagement.
Mugabe’s government says the cholera outbreak is a calculated attack by former colonial ruler Britain and the United States which have used "biological warfare" to create an excuse to mobilise military action against Zimbabwe.