* Case challenged legitimacy on upper house of parliament
* House to continue working until election, expected in April
* Delay eases immediate pressure on president
CAIRO, Jan 15 (Reuters) - Egypt's top court postponed on Tuesday a ruling on a case that challenged the legitimacy of the upper house of parliament and threatened further political uncertainty for the nation and its troubled transition to democracy.
The decision means the upper house, which has lawmaking power in the absence of a lower house, can continue working for now and could ensure its stays in place until a new lower house is elected in an election expected to start in April.
It also lifts the immediate pressure from President Mohamed Mursi, who would have had to take back legislative powers had the upper house been declared invalid, opening him up to the possibility of renewed criticism that he is monopolising power.
Egypt's transition to democracy since the fall of Hosni Mubarak two years ago has been plagued by street violence and a slew of complex legal wrangles - frustrating ordinary Egyptians and unnerving investors, taking a heavy toll on the economy.
The head of the Supreme Constitutional Court, Maher el-Beheiry, told reporters that the case would be referred back to an advisory legal panel to review. He did not say when that panel would issue a recommendation or when the court would rule.
The case was raised by independent lawyers opposed to Mursi and his fellow Islamists who dominated both houses. The court's declaration last year that the lower house was void led to that assembly being dissolved.
Subsequently, Mursi temporarily added legislative powers to his executive role, prompting opponents to accuse him of hogging power. Lawmaking powers only moved to the upper house after a new constitution was approved in a December referendum.
For now, it is not clear which way the court will rule when it does finally issue a decision.
Although the upper house was elected based on the same election law that the court found lacking when it declared the lower house void, one expert said the new constitution changed the legal landscape and said this could undermine the case.
The make-up of the Supreme Constitutional Court, the highest in the land, has also changed since the ruling against the lower house because the new constitution cut the number of judges on the panel and changed the rules for their appointment.
Islamists had said the previous court panel was stuffed with Mubarak-era loyalists. But the changes under the new constitution have meant one outspoken critic of Mursi and others who Islamists saw as close to the old order were removed from the Supreme Constitutional Court panel.
Another case challenging the make-up of the assembly that drew up the newly approved constitution was also before the court on Tuesday. The court postponed that case to a session on Feb. 3.
Liberals walked out of the Islamist-dominated constituent assembly saying their voices were not being heard.