* Ravalomanana threatens to boycott next week's talks

* AU says presidency not up for discussion

(Recasts with Ravalomanana, adds details, background)

By Alain Iloniaina

ANTANANARIVO, Oct 28 (Reuters) - Prospects for a deal to end Madgascar's political crisis looked grim on Wednesday after a former leader refused to negotiate unless President Andry Rajoelina stepped aside.

The African Union said earlier on Wednesday that Madagascar's political leaders would meet next week in Ethiopia in another attempt to hammer out the details of a power-sharing deal aimed at restoring constitutional order.

Rajoelina, who at 35 is Africa's youngest leader, unseated Ravalomanana in a coup after weeks of popular protests against the former leader's alleged abuse of power.

"It is non-negotiable. I will never accept Rajoelina as president of the transition," Marc Ravalomanana, the former leader ousted in March, told French newspaper Le Monde.

The African Union has said the post of president was not up for discussion at the latest meeting, slated for Nov. 3-5 in Ethiopia's capital, Addis Ababa.

Ravalomanana, a self-made millionaire now exiled in South Africa, said he would stay there if there was no need for him at the negotiating table.

Ravalomanana opposed a reshuffle of key posts earlier this month, endorsed by the international community, which saw Rajoelina retain the presidency and a relatively unknown social anthropology professor picked as prime minister.

NOT FOR DISCUSSION

AU special envoy Ablasse Ouedraogo said the Ethiopia meeting had the sole objective of finalising the make-up of a consensus government agreed upon by the country's power-brokers in August.

"Andry Rajoelina is the president of the transition. There is no dispute as far as the international community is concerned," Ouedraogo told reporters late on Tuesday.

Ouedraogo said he expected all leaders -- Rajoelina, Ravalomanana and former presidents Didier Ratsiraka and Albert Zafy -- to attend the meeting.

Some analysts say Ravalomanana's hard-line stance is leaving him increasingly isolated.

But the business tycoon has argued that endorsing Rajoelina would legitimise an unconstitutional power-grab. He told the French daily it was Rajoelina's turn to make concessions.

"The international community pushed me not to take part in the transitional government, even though I am the elected president. The putschist Andry Rajoelina must do the same and quit his post as president of the transition," he said.

"It is up to the mediators to do their work," he said.

Under the terms of August's power-sharing deal, a presidential poll must be held by late 2010. Rajoelina has called for elections by April next year. (Writing by Richard Lough; editing by Robin Pomeroy)

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