BISSAU (Reuters) - Guinea Bissau's dominant political party on Monday rejected President Jose Mario Vaz's newly-appointed prime minister and said it would refuse to join his new government, likely extending a year-long political paralysis.

Vaz had to dissolve the previous government after the last prime minister, Baciro Dja, failed to win the full support of his ruling African Party for the Independence of Guinea and Cape Verde (PAIGC), an organization hit by regular infighting

Political rivals in September agreed to a roadmap to form a consensus government with a view to ending the crisis that has frozen international aid and shut parliament. [nL8N1BN091]

However, the PAIGC said it would not join the government of Umaro Mokhtar Sissoco Embalo, a brigadier general who was appointed the West African country's fifth prime minister in a little over a year on November 18. [nL8N1DJ4ZT]

"The leadership of the PAIGC reaffirms its irrevocable intention not to take part in the government of Umaro Sissoco," the statement said, adding that President Vaz was the "principal factor in the political crisis".

The PAIGC, of which Vaz is a member and whose original popularity comes from its role in defeating the Portuguese colonizers in a civil war that ended in 1974, is led by the current president's arch rival Domingos Simoes Pereira.

The latter, who built his reputation among diplomats and investors as a reformer, was dismissed as prime minister by Vaz in August 2015 amid a growing personal rivalry between the two men.

The PAIGC statement means that Sissoco could struggle to convene parliament after he appoints a cabinet, which would prevent a budget from being approved.

So far, the political crisis in the coup-prone former Portuguese colony has remained confined to the political elite with the army remaining neutral.

It has seen nine coups or attempted coups since 1980 and instability has helped it become a major transit point for cocaine trafficked from South America to Europe.

(Reporting by Alberto Dabo; Writing and additional reporting by Emma Farge; editing by Ralph Boulton)