China´s Shenzhou VI spacecraft is not orbiting exactly as planned and will have to be restored to its original trajectory, state-run media say.
The "orbit maintenance operation" would take place early on Friday morning, said official news agency Xinhua.
Gravity has drawn Shenzhou VI too close to earth, the agency said.
Shenzhou VI, which has two astronauts on board, is in a low enough orbit to be affected by the Earth´s gravitational pull.
It spent a second day in orbit on Thursday, making it China´s longest manned space flight.
Xinhua quoted experts as saying the procedure to fix craft´s orbit would be a "normal technical operation".
Nonetheless, the agency said, experts were urging all scientific and technological staff to be "cautious".
Everything was functioning well aboard Shenzhou VI and it had orbited the earth 23 times by Thursday evening, according to Xinhua.
Astronauts Fei Junlong and Nie Haisheng, who is marking his 41st birthday on Thursday, were reportedly in good health.
CHINA´S SPACE PROGRAMME
1958: Base for space flights built at Jiuquan, in Gobi desert
April 1970: First satellite launched
Oct 2003: First manned space mission on Shenzhou V
Oct 2005: Shenzhou VI takes two men into space
They spent the day carrying out experiments on the craft´s stability.
It was a unique birthday for Mr Nie, whose daughter sang Happy Birthday to him late on Wednesday.
"It´s marvellous around here. The Earth looks beautiful," Mr Nie told 11-year-old Tianxiang.
His wife, Wang Jie, said: "May you carry out the task entrusted to you by the motherland and return smoothly." Xinhua said that "at these words, Nie Haisheng was in tears".
Washington on Wednesday congratulated China on a successful launch.
Fei Junlong, 40 (left), and Nie Haisheng, 41
Both married with one child
Formerly in the air force - have more than 3,200 hours flying time between them
Joined space training programme in ´98
Dining on Chinese specialities in space but chopsticks too difficult to use
US space agency Nasa also said the latest mission reaffirmed China´s place, along with the US and Russia, in the space "elite".
The mission is expected to see the Shenzhou VI orbit the Earth for up to five days, during which the astronauts will carry out experiments.
It comes almost exactly two years after China´s first manned space flight.
China´s official media has speculated that the space capsule will circle the Earth up to 80 times before landing in the Chinese region of Inner Mongolia.
Beijing has attached great importance to its space programme, viewing it as a source of national pride and international prestige.
China hopes to set up a space station within five years and eventually it wants to put an astronaut on the moon.
CHINA´S SHENZHOU SPACECRAFT
1. Forward orbital module - crew live and work in this section, which contains scientific equipment. In future missions, this module may remain in space as part of a Chinese space station 2. Re-entry capsule - contains seats for three crew 3. Propulsion module - contains spacecraft´s power unit and liquid fuel rocket system 4. Solar panels - spacecraft carries two sets with a total area of 40 square metres, which generate an average 1.5kW of power