Egyptian President Mohammed Mursi's order to reconvene parliament has been rejected by the highest court, which says its ruling that led to the assembly's dissolution is binding.
The speaker of the dissolved house has already responded to Mr Mursi's decree, calling on MPs to meet on Tuesday.
Army units outside parliament have left and some MPs have gone in.
The decision by Mr Mursi, whose Muslim Brotherhood has most seats, sets up a potential showdown with the military.
However the situation is unclear as Egyptians elected Mr Mursi without a constitution and without his powers being defined.
Because parliament has been dissolved, he was sworn in before the constitutional court rather than before MPs.
It was the Supreme Council of the Armed Forces (Scaf) that made the decision to dissolve parliament in June, after Egypt's Supreme Constitutional Court ruled that part of the election for parliament was unconstitutional.
Meeting on Monday, the court said that all its rulings and decisions were "final and not subject to appeal".
In a statement, it emphasised that the court was "not a party to any political confrontation".
The BBC's Jon Leyne in Cairo said that the court had not itself ordered the dissolution of parliament so Mr Mursi was not directly challenging a court order.
While MPs could meet, any laws they passed were most likely to be ruled invalid, he said.
Despite the apparent tensions, the president and Field Marshal Hussein Tantawi, who heads Scaf, appeared together at a military cadet graduation ceremony on Monday.
Parliament speaker Saad al-Katatni, also a member of the Brotherhood, said MPs should return for a session of parliament on Tuesday afternoon.
A Salafist MP, Nizar Ghurab, was the first to go into the building as guards outside allowed MPs to return, Mena news agency reported.
But Mr Mursi's decree was criticised by some of his political rivals.
Presidential candidate Hamdin Sabbahi was quoted as saying it was a "waste of legal authority" while another, Abdul Moneim Aboul Fotouh, was said to have denounced his move as unconstitutional.
Liberal MP Mohammed Abu Hamed urged Scaf to challenge what he called "this constitutional coup".
The constitutional court says it has received a number of appeals against the president's order. The head of the court Maher el-Beheiry told Reuters news agency it would hear the cases on Tuesday.
The Scaf took over the reins of power last year, after the revolution that ended former President Hosni Mubarak's 30-year rule.
The army move was initially welcomed by many of the anti-government protesters, but its presence became increasingly unpopular as critics accused its leaders of wanting to hold on to power.
Mr Mursi won the country's first free presidential election last month, and army chiefs formally handed over power on 30 June.
But before his inauguration, the military granted itself sweeping powers.
The commanders' constitutional declaration stripped the president of any authority over the military, gave military chiefs legislative powers, and the power to veto the new constitution, which has yet to be drafted.
In his decree, Mr Mursi said new parliamentary elections would be held 60 days after the new constitution had been agreed by referendum.
The Muslim Brotherhood has consistently opposed the decision to dissolve parliament.