CAIRO (AP) - Egypt's highest court ruled on Sunday that the nation's Islamist-dominated legislature and constitutional panel were illegally elected, dealing a serious blow to the Islamists' hold on power and prolonging the political instability that has gripped the country since the ouster of autocrat Hosni Mubarak two years ago.

But the ruling by the Supreme Constitutional Court also said that the legislature's upper house, the only one currently sitting, would not be dissolved until the parliament's lower chamber is elected later this year or early in 2014.

The same court ruled to dissolve parliament's lower chamber in June, a move that led to the promotion of the toothless upper chamber, the Shura Council, to becoming a law-making house. The Shura Council, long derided as nothing more than a talk shop, was elected by about seven percent of the electorate last year.

It was not immediately clear whether the ruling on the 100-member constitutional panel would cancel the charter it drafted. The constitution was adopted in a nationwide vote in December with a relatively low turnout of about 35 percent.

But even if it does not, the ruling will question the legitimacy of the disputed charter pushed through by allies of Islamist President Mohammed Morsi in an all-night session late last year. Critics say the charter restricts freedoms and gives clerics a say in legislation. The Islamists who drafted it hail the document as the best one Egypt ever had.

There was no immediate comment from Mr Morsi's office on the ruling.

Regardless of their consequences on the ground, Sunday's ruling is likely to deepen Egypt's protracted political instability. It will give heart to the mostly secular and liberal opposition, while providing fresh ammunition to the argument often repeated by the president's supporters that the judiciary is filled with Mubarak loyalists determined to derail the nation's political process.

Mr Morsi, elected nearly a year ago, tried to reinstate parliament's lower chamber just days after he came to office on June 30 but eventually bowed to the court ruling and backed down.

In both rulings on parliament's two chambers, the court contended that political parties that fielded candidates for the third of seats set aside for independent candidates, as allowed by the election law, amounted to a breach of the principle of fairness.