Venezuela's Supreme Court has ruled that the postponement of President Hugo Chavez's inauguration for a new term in office is legal.
Earlier, the National Assembly voted to give Mr Chavez as much time as he needed to recover from cancer surgery.
The opposition argues that Mr Chavez's current mandate expires on 10 January, the day he is due to be sworn in.
Mr Chavez is in hospital in Cuba and has suffered complications caused by a lung infection.
Supreme Court President Luisa Estella Morales said it would be "absurd" to consider Mr Chavez's treatment in Cuba as an unauthorised absence.
She said that the oath of office could be taken at a later date, as the constitution states, and Mr Chavez's re-election had guaranteed "continuity" in government.
In a televised statement, Ms Morales read out the unanimous decision of the panel of seven magistrates who handle constitutional issues before the court.
She said there was a clear distinction between the act of taking the oath and the beginning of a new mandate.
The Supreme Court considered the swearing-in an important formality, but not indispensable for the start of the new presidential term, she said.
According to the opposition, Mr Chavez should be declared temporarily incapacitated with the current Speaker of the National Assembly, Diosdado Cabello, not Vice-President Nicolas Maduro, taking over as caretaker leader.
They argue that Mr Maduro, who was appointed by Mr Chavez not elected, will cease to be vice-president on 10 January.
But Ms Morales said that "the executive power made up of the president, vice-president, ministers and other organs, as well as administration officials, will continue to carry out their functions under the principle of administrative continuity".
She said that the swearing-in ceremony would no doubt happen.
"But at this moment, we can't say when, how, or where he [the president] will be sworn in", Ms Morales said.
President Chavez, who has been in power since 1999 and was re-elected in October for a fourth term, has not been seen or heard in public for almost a month.
Mr Cabello has called on Chavez supporters to take to the streets of Caracas on Thursday to show support for him.
He said several foreign leaders had agreed to be at the Miraflores Presidential Palace on inauguration day.
BBC News, Caracas
Never before has a Venezuelan president missed his swearing-in ceremony. And yet, Venezuelans seem to be no nearer to knowing when their president will return.
The Supreme Court has ruled his inauguration can be delayed indefinitely, with a new date to be set once his health problems have been resolved. Earlier, the National Assembly had said Mr Chavez could take as long as he needed to recuperate.
That means the present situation, with Vice-President Nicolas Maduro acting as head of state in Mr Chavez's absence, could go on indefinitely.
Opposition groups have made clear their disagreement with this arrangement but they've run out of options to challenge it within Venezuela. They've already contacted the Organisation of American States (OAS) to air their grievance, but it's unlikely this body, which has had a poor relationship with Mr Chavez's government, could influence the situation.