Zimbabwe’s Prime Minister Morgan Tsvangirai says there is too little time to meet the 31 July deadline for national elections.
Key reforms to guarantee free and fair elections still needed to be implemented, he added.
President Robert Mugabe said last week he would abide by the deadline set by the Constitutional Court.
The elections would herald the end of the coalition the two leaders formed in 2009 after disputed polls.
Mr Tsvangirai’s Movement for Democratic Change (MDC) joined Mr Mugabe’s Zanu-PF party in a power-sharing government formed under pressure from regional leaders to end almost a decade of instability in Zimbabwe.
The unity government has helped end the hyperinflation that saw Zimbabwe’s economy collapse, but the administration has been fraught with squabbles over introducing reforms.
Meanwhile, the New York-based campaign group Human Rights Watch (HRW) has said there is an urgent need to reform Zimbabwe’s army, police and intelligence agencies ahead of elections.
‘Elephant in room’
Mr Tsvangirai failed to secure the presidency in elections in 2002 and 2008, following widespread violence and allegations of vote-rigging by Zanu-PF.
He is expected to challenge Mr Mugabe again in this year’s election – the first under a new constitution which expands civil liberties.
Mr Tsvangirai said Mr Mugabe had adopted a “dishonest position” by insisting that the court’s ruling be upheld.
“In 2002 we went to elections and I challenged the result. Up to now, the court hasn’t even heard the case. So where is the rule of law, the respect that he’s talking about?” he said.
The prime minister was speaking in the capital, Harare, after meeting the leaders of other parties which have broken away from the MDC and Zanu-PF.
It was the first such meeting they held, suggesting that they were trying to form a common front, reports the BBC’s Brian Hungwe from Harare.
The deputy leader of a breakaway MDC faction, Edwin Mushoriwa, said the court should reconsider its ruling to avoid a repeat of the conflict that hit Zimbabwe during the 2008 poll.
The HRW report entitled The Elephant in the Room said that some security chiefs had already declared allegiance to Mr Mugabe ahead of the polls.
“With the security forces right up to the top leaders threatening and attacking Mugabe’s perceived opponents, Zimbabweans have little faith in the upcoming elections,” said Tiseke Kasambala, HRW’s Africa advocacy director.
“Zimbabwe’s unity government is going to have to rein in the security forces and keep them out of politics if the elections are going to have any meaning.”
Zimbabwe Defence Forces commander Gen Constantine Chiwenga was among top security officials who had made disturbing comments, HRW said.
On 4 May, he told Zimbabwe’s state-run Sunday Mail newspaper that he would not hold talks with Mr Tsvangirai to discuss security reforms.
“We have no time to meet sell-outs. Clearly Tsvangirai is a psychiatric patient who needs a competent psychiatrist,” Gen Chiwenga was quoted as saying.
Zimbabwe’s Constitutional Court said on Friday that elections must be held by 31 July, and that Mr Mugabe should set a date “as soon as possible”.
State-controlled Zimbabwe Broadcasting Corporation (ZBC) quoted Mr Mugabe as saying he would comply with the ruling and the coalition had “outlived its usefulness”.
Five years ago, Mr Tsvangirai won the most votes in the first round of the presidential election but, according to official results, not enough to win outright.
He pulled out of the second round, saying his supporters were being targeted in a campaign of violence.
After Mr Mugabe went ahead with the election, winning with 85% of votes cast, regional mediators intervened to organise the power-sharing agreement.