Sudan opposition Umma party will boycott elections

* Umma had demanded four-week delay in elections

* Move blights credibility of elections

* Party thinks others may follow suit

(Adds details, background)

KHARTOUM, April 7 (Reuters) - Sudan's Umma Party, one of the main opposition parties, said on Wednesday it would boycott next week's presidential, legislative and gubernatorial polls, blighting their credibility.

President Omar Hassan al-Bashir had hoped to win the April 11 polls in defiance of an International Criminal Court warrant for his arrest to legitimize his rule in Africa's largest country.

The withdrawal of the Umma party, and that of the main south Sudan party a day earlier, will cast a major shadow over the elections, Sudan's first multi-party polls in 24 years.

"The political bureau decided to boycott the current elections at all levels," senior Umma party official Sara Nugdullah told reporters.

The party had set eight conditions for participation including a four-week delay and government funding for political parties. Only one condition, a campaign spending ceiling, was met by the April 6 deadline.

Nugdullah said Umma party leader Sadeq al-Mahdi had been granted the right to "take action in the national interests," but three party officials said this would not affect the decision to boycott the elections.

Two party sources said Mahdi may consider taking a position similar to that of the ex-southern rebel Sudan People's Liberation Movement (SPLM), which on Tuesday announced a boycott in the north, except in the central Blue Nile and South Kordofan states.

Mahdi was the last democratically elected leader of Sudan in 1986 and was one of Bashir's two main challengers in the presidential polls.

The favourite to threaten Bashir, SPLM candidate Yasir Arman, withdrew last week citing major fraud and the continuing conflict in Darfur, sparking a crisis of confidence in the elections and leaving a loose opposition alliance in disarray.

On Wednesday the biggest international observer mission -- from the European Union -- said it was withdrawing its observers from war-torn Darfur because fighting and kidnappings were restricting the movement of its staff, undermining their ability to observe election preparations.

The Communist Party, the Umma breakaway Reform and Renewal and other smaller parties announced a full boycott last week.

The polls, due to begin on Sunday, look shaky and Umma Party political bureau member Ahmed Abdallah said the Umma decision may persuade other parties still contesting the elections to join their boycott. (Reporting by Opheera McDoom, editing by Tim Pearce) (For FACTBOX on opposition complaints about elections, click on [ID:nHEA656138]; for FACTBOX on mechanism of Sudan's complex elections, click on [ID:nHEA656488])