BISSAU, April 9 (Reuters) - Guinea-Bissau's president rejected a consensus prime minister proposed by a broad coalition of political parties on Monday, prolonging a political crisis in one of the world's poorest nations.

The coalition, formed last month by two ruling and one opposition party, wants a prime minister backed by all three parties to replace a presidential ally who resigned on March 29 after the three won a no-confidence vote against him.

A spokesman for the coalition said President Joao Bernardo "Nino" Vieira had rejected its candidate for the premiership, Martinho Ndafa Kabi of the opposition African Party for the Independence of Guinea-Bissau and Cape Verde (PAIGC).

Former Prime Minister Aristides Gomes, a close ally of Vieira, resigned on March 29 after the no-confidence vote against his government in the former Portuguese colony.

The PAIGC and the two ruling parties, the Social Renewal Party (PRS) and the United Social Democratic Party (PUSD), signed the National Political Stability Pact last month.

The three "will maintain the name of Martinho Ndafa Kabi as successor to Aristides Gomes," said coalition spokesman Ibrahima Sori Diallo. "We do not know when a new prime minister will be appointed."

The three control almost all parliamentary seats and say Guinea-Bissau, which has been rocked by coups and civil wars since independence in 1974, can only progress under a government with a legislative majority.

The parties have in the past threatened street protests unless Vieira accepts a premier proposed by them.

Vieira could call early parliamentary elections but diplomats say this is unlikely as the cash-strapped state, dependent on cashew nut production, does not have the money to organise them.

Vieira ruled for nearly 20 years after toppling Guinea-Bissau's first president, Luis Cabral, in 1980. He was overthrown in a crippling 1998-1999 civil war, but won an election in 2005, after which he installed Gomes as premier.

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