Mr Tsvangirai wants the president to cede real executive power to him
Zimbabwe's main opposition party has said two days of power-sharing talks with the ruling Zanu-PF have ended without agreement.
Movement for Democratic Change (MDC) spokesman Nelson Chamisa told the BBC the balance of power was in dispute.
He said President Robert Mugabe wanted MDC leader Morgan Tsvangirai to become a titular prime minister without real authority, which was "unacceptable".
Mr Mugabe last week threatened to form a new government without the MDC.
"The MDC does not want to come in apparently," he said on Wednesday, a day after being booed and jeered by opposition MPs at the formal opening of parliament.
Following legislative elections in March, Mr Mugabe's Zanu-PF lost its majority in the House of Assembly for the first time since independence in 1980.
Mr Tsvangirai also won the first round of the presidential election that month, before pulling out of the run-off in June citing a campaign of violence against his supporters.
The MDC said its negotiating teams had returned from South Africa on Sunday without having achieved a breakthrough in the talks on Zimbabwe's political crisis.
"Nothing was achieved in the latest round of engagement in South Africa to break the deadlock. We remain where we were," Mr Chamisa told the Reuters news agency.
Mr Chamisa later told the BBC that the MDC would not accept the current offer from Zanu-PF for the opposition leader to be only a titular prime minister.
Correspondents say both sides in the negotiations have agreed that there should be a national unity government, that Mr Mugabe should be president and Mr Tsvangirai prime minister.
But the MDC has insisted that the president should cede real executive power to Mr Tsvangirai and stay in office only as a ceremonial head of state.
Earlier, the state-owned Herald newspaper reported that the MDC had also proposed that the two leaders chair the cabinet jointly.
"The Zanu-PF dismissed the suggestion, not just as insolent, but also stunning ignorance on how government works," it quoted a government source as saying.
Meanwhile, Mr Chamisa welcomed the government's decision to lift the ban on aid agencies involved in providing food and other forms of assistance. He said the restriction had been an act of madness.
The government had accused some of the agencies of siding with the opposition in the run-up to the presidential election's second round.