Egypt's Islamist-dominated parliament was set to convene on Tuesday in defiance of the powerful military following a decree by newly elected President Mohamed Morsi to reinstate the assembly.
Parliament speaker Saad al-Katatni has invited members of the lower house to meet at 2:00 pm (1200 GMT), but Egypt's highest court ruled on Monday that the decree reinstating the lower house was unconstitutional.
"All the rulings and decisions of the Supreme Constitutional Court are final and not subject to appeal... and are binding for all state institutions," said the court.
The Supreme Council of the Armed Forces, which handed power back to Morsi last month after he was elected, backed the court, saying the rule of law must be upheld.
The SCAF, which ruled Egypt after former president Hosni Mubarak was ousted in a popular uprising last year, underlined the "importance of the constitution in light of the latest developments," the official MENA news agency reported.
Islamists scored a crushing victory in three-stage parliamentary elections held from November last year, with the Muslim Brotherhood, Morsi's former organisation, heading the lower house.
But the military dissolved parliament last month after the top court made its controversial ruling a day before the second round of the presidential poll that saw Morsi become Egypt's first democratically elected head of state.
The Supreme Constitutional Court had said certain articles in the law governing the parliamentary elections were invalid, annulling the Islamist-led house.
But Morsi on Sunday ordered the lower house to reconvene.
The Muslim Brotherhood said it "will participate (Tuesday) in a million-man march in support of the president's decision and reinstating parliament."
The presidency insisted the decree "neither contradicts nor contravenes the ruling by the constitutional court."
The ruling does not need to be implemented immediately, said presidential spokesman Yasser Ali, arguing Morsi's decision "takes into account the higher interest of the state and the people."
The confrontation prompted the United States to urge Egypt to respect "democratic principles."
US secretary of state Hillary Clinton is due in Egypt on July 14 to express US support for the process of democratic transition in the US ally state.
Washington lavishly supported Mubarak during his 30 years in power but analysts say US officials will now have to work with multiple centres of power -- including a military seen as restricting Morsi's room for manoeuvre.
During a visit to Cairo, German Foreign Minister Guido Westerwelle held out the prospect of fresh investment and trade if Egypt continues on the road of democratic progress.
The constitutional court stressed it was "not a part of any political conflict... but the limit of its sacred duty is the protection of the texts of the constitution."
The SCAF also insisted the texts of the constitution must be upheld.
It was not clear how the court's ruling would be enforced.
Morsi's decision caused a "political earthquake," some media reported on Monday, and it also angered some secular parties which had slammed the Muslim Brotherhood's monopolisation of power since the start of the uprising.
"In any decent and democratic country, a president cannot disrespect the judiciary," said Rifaat al-Said, head of the leftist Al-Tagammu party.
"Whether Morsi likes it or not, he must respect the judiciary's decisions," he told state television.
After parliament was annulled last month, the SCAF issued a constitutional declaration granting the military sweeping powers, and in the absence of a parliament -- in which nearly half of seats were won by the Brotherhood and another quarter by hardline Salafists -- it assumed legislative power.
SCAF's document, which rendered the presidency toothless, caused outrage among those calling for the military to return to barracks.
Instead of being sworn in before parliament, the 60-year-old Morsi took the oath on June 30 before the constitutional court.