* Constitutional court to hear case against upper house
* Ruling could create legislative vacuum until new elections
* Case pits lawyers against Islamists in post-Mubarak Egypt
By Tamim Elyan
CAIRO, Dec 30 (Reuters) - Egypt's highest court is to examine the legitimacy of the upper house of parliament, a case likely to increase the legal uncertainty of the country's political transition and leave it without a legislature.
The Supreme Constitutional Court has already forced the lower house to dissolve under a June ruling that said an electoral law used to elect both chambers - and which resulted in an Islamist dominated parliament - was unfair.
The new case once again pits independent lawyers against the Islamists that have come to power since the downfall of Hosni Mubarak and who see the court as stuffed with the former president's appointees.
On Sunday, at its first session since going on strike over President Mohamed Mursi's decision to expand his powers, the court set Jan. 15 as the date for the first hearing. It will also examine the legitimacy of the Islamist-dominated assembly that wrote the constitution that Mursi fast-tracked to approval at a referendum this month.
If the court rules against the upper house - which seems likely as it was elected under the same law that the court found lacking in the lower house case in June - Egypt will have a legislative vacuum until new parliamentary elections, expected to start in about two months.
The upper house assumed legislative powers just last week under the new constitution.
The June ruling said the transitional electoral law - drafted by the generals then ruling post-Mubarak Egypt in consultation with political parties - gave too much power to parties, at the expense of independent candidates.
The court had to postpone the hearings from early December due to a protest outside its building by Islamists.
"The (court) renews its condemnation of those who participated in it, as well as those who remained silent," the court said in a statement.
Mursi had shielded both the assembly which drafted the constitution and the upper house of parliament from legal challenges in a decree he issued in November which touched off protests by critics who accused him of a power grab.
Mursi revoked the decree ahead of the referendum in which the constitution was approved by 64 percent of those who voted.
The new constitution cut the number of judges who sit in the Supreme Constitutional Court to 11 from 18. One of Mursi's most vocal critics was among the judges to leave the court. (Editing by Tom Perry and Robin Pomeroy)