CAIRO, May 12 (Reuters) - Egypt's highest court will rule next month on the legality of the upper house of parliament, judicial sources said on Sunday, in a case that throws more legal doubt over a political transition repeatedly disrupted by lawsuits.

A ruling against the Muslim Brotherhood-led upper house by the Supreme Constitutional Court on June 2 could lead to the dissolution of the chamber and result in legislative power being transferred to President Mohamed Mursi.

The case, brought by an independent parliamentary candidate, stems from a similar challenge to the lower house last year. That lawsuit resulted in the dissolution of the Islamist-led lower house after the court ruled that flaws in the election law, related to the number of seats reserved for independent candidates, rendered it unconstitutional.

Both are examples of the many lawsuits, filed mostly by Muslim Brotherhood opponents, that have complicated the transition from Hosni Mubarak's rule after he was swept from power by Egypt's 2011 uprising.

Elections for a new lower house of parliament were due to begin last month but were derailed by a court ruling annulling a Mursi decree setting the election dates.

The Islamists believe the judiciary to be stacked with Mubarak-era appointees who are working against them. However, the make-up of the Supreme Constitutional Court has changed since last year's ruling on the dissolution of the lower house.

The court is also due to rule on June 2 on the validity of a separate Islamist-dominated assembly that drafted a new constitution fast-tracked into law by Mursi in December.

Many Egyptians viewed the assembly as illegitimate because it did not fully represent the country's diversity. Most of the non-Islamist members, including liberals and representatives of the Coptic Church, withdrew, saying that Islamists were imposing their views.

The constitution was approved in a popular referendum, which some critics say offers it protection from legal challenge.

(Reporting by Maggie Fick; Editing by Tom Perry and David Goodman)