The divisive deal that Robert Mugabe struck to keep himself in power was in question yesterday as Morgan Tsvangirai, the main opposition leader, quit talks in the hope of more concessions.

Tsvangirai, the leader of the Movement for Democratic Change, which won a parliamentary majority in March, appears to reckon that it is irrelevant that Mugabe has split the MDC, doing a deal with a small faction to give himself nominal control of parliament once more. Tsvangirai's calculation is that no deal that lacks his signature is worth anything; nor will aid and international recognition come back until he is given a real role in the government.

He may be overplaying his hand. Yesterday he was described by those close to the talks as very buoyant, believing that he had put the ball in Mugabe's court. But in a game of bluff which is far from over, he is reckoning that the world thinks he is indispensable to Zimbabwe's future. He may be right, but that doesn't mean that Mugabe will recognise this as pressure, and respond in the way that Tsvangirai - and much of the world - is hoping.

Mugabe's latest manoeuvre to keep himself in power came to a climax on Tuesday night, when he struck a deal for a coalition government with Arthur Mutambara, leader of a faction of the MDC, in talks brokered by President Mbeki of South Africa. On paper, this would give Mugabe's Zanu (PF) party, together with Mutabara's MDC faction, 109 seats to Tsvangirai's 100.

But many agree with Tsvangirai that “the world will not re-engage with Zimbabwe until the MDC is on board and really has a role,” as Richard Dowden, Director of the Royal African Society, said. “A deal stitched up by Mugabe, Mutambara and Mbeki isn't going to stick,” he added.

That is clearly the calculation that Tsvangirai has made, despite Mbeki's attempts to present this as progress. He has been buoyed up by the blast of international support for his demand, which, in its simplest form, is to be made prime minister with executive power, leaving Mugabe as something of a figurehead. Tsvangirai may travel to South Africa before this weekend's regional summit to try to convince Mbeki of this requirement of legitimacy.

But others take a cooler view of the strengths of his position. “Neither MDC faction is negotiating from a particularly strong position,” argues Tom Cargill, manager of the Chatham House think-tank's Africa Programme. “They have none of the levers of power within Zimbabwe, in the military or economic spheres.”

At the moment, the Army has thrown its support behind Mugabe, in line with the self-interest of its senior members. They want protection of their commercial interests, as well as protection from the potential charges a new president might bring. Although they might, in theory, be persuaded to back someone new, at the moment they clearly do not.

The ideal, Cargill argues, is for Tsvangirai to be given the executive powers he wants. But “the best we can probably hope for,” he added, “is some cobbled-together agreement which needs to include Tsvangirai but might not be substantive.” It would, however, have to include a promise from Mugabe to work within parliamentary processes, something that many Zanu (PF) members want too as they do not want the Army calling the shots. That would allow Mugabe to retire soon by choice, and to maintain that he was not forced out, and that his legacy was safe. “Zimbabwe needs a younger generation of politicians from all parties to be given a chance to sort out the mess,” Cargill said. “While Mugabe is there, it won't be.”

Even that sketch remains out of reach at the moment, as Mugabe has offered nothing like that in these talks. Any movement will probably have to wait for next week. Not much is likely to happen before the regional summit in South Africa this weekend, chaired by Mbeki (though Tsvangirai may try to pitch up there first to bend the South African leader's ear). Nor is there going to be any more talk of sanctions in the UN unless Tsvangirai says that he can get no satisfactory concessions from these talks.

The next week will begin to show whether Tsvangirai is justified in his confidence in using international support as a bargaining chip in wrestling with a leader who has never attached much value to that commodity.

Times Recommends

• Michael Jackson: Not the final chapter
• Indian Army spends10m rupees on golf buggies
• Brüno toned down for 15 rating