* South expected to choose independence in January vote
* Gaddafi and Mubarak to meet Bashir Tuesday-state media
KHARTOUM, Dec 20 (Reuters) - Sudan's highest court has thrown out one legal challenge to a referendum on southern independence, but is considering two others that could derail the vote scheduled for next month, officials said on Monday.
Southerners are now 20 days away from the scheduled Jan. 9 start of the plebiscite on whether to declare independence or stay in Sudan, a vote promised in a 2005 peace deal that ended decades of civil war with the north.
Preparations for the referendum -- which are already far behind schedule -- have been disrupted by last-minute petitions to Sudan's Constitutional Court from groups citing a string of irregularities and calling for the organising commission to be dissolved and voter registration to be re-run.
Southerners, who are widely expected to choose independence, have accused the north of backing the legal challenges to keep control of the south's oil reserves, accusations dismissed by the north's ruling National Congress Party (NCP).
Analysts have warned that there is a risk of a return to conflict if the vote is disrupted or cancelled. <^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ For other referendum stories, click on [ID:nLDE6BD0GS] ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^>
An official at the Constitutional Court, who asked not to be named, told Reuters that judges had thrown out one case and were looking into two other petitions. The official declined to go into the detail of the cases.
Two groups, called the Society Organisation Network and the Higher Council for Peace and Unity, have announced legal challenges. Their representatives and lawyers were not immediately available for comment on Monday.
The head of the referendum's organising commission Mohamed Ibrahim Khalil said the court had asked him to respond to points raised in one of the surviving legal challenges.
"The court has summarily dismissed one case. We have received details of another petition and we have been asked to reply to it. That is what I am doing now," he said.
"From what I have seen so far there is absolutely no substance to these petitions. Some are ridiculous. One of them said that the CPA was unconstitutional," he added, referring to the 2005 Comprehensive Peace Agreement that set up Sudan's interim constitution and promised the referendum.
The referendum commission was sworn in this July, about three years late. Khalil said he was expecting ballot papers to arrive late on Wednesday or early on Thursday which, he insisted, still gave organisers just enough time to distribute them to remote parts of the south before the Jan. 9 deadline.
International observers from the Carter Centre last week praised the commission for holding a "generally credible" registration of voters.
Veronique De Keyser, who heads the European Union observer mission, said on Monday that the registration had been "comprehensive and generally peaceful" but told reporters she would give her full judgement after the voting took place.
Libyan leader Muammar Gaddafi and Egyptian President Hosni Mubarak are due to visit Khartoum to meet Sudanese President Omar Hassan al-Bashir on Tuesday, Sudan's state Suna news agency reported.
Diplomatic sources in Khartoum said they were expected to discuss plans for the referendum and the implications of a north-south split. (Reporting by Andrew Heavens and Khaled Abdel Aziz; editing by David Stamp)
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