The 84-year-old, appointed prime minister by King Gyanendra Thursday evening, was scheduled to be administered the oath of office at the Narayanhity palace at 11 a.m. Friday, two hours before parliament convenes after four long years.
However, the frail leader, who had been suffering from severe respiratory problems and been on oxygen and saline Thursday, was advised rest by his physicians.
Krishna Sitaula, spokesperson of Koirala's Nepali Congress party, told IANS that the new prime minister's health had improved and doctors were expected to be able to say when he could resume normal activities by late afternoon.
The five-time prime minister did not attend the massive rally in the capital Thursday held to celebrate the victory of the people's peaceful movement for 19 days that forced King Gyanendra to relinquish power.
Parliament would convene according to schedule but without Koirala.