The launch of the James Webb space telescope scheduled for December 22 won't take place before December 24, NASA announced on Wednesday.
The NASA project, launched in 1989, was originally expected to deploy in the early 2000s.
But multiple problems forced delays and a tripling of the telescope's original budget with a final price tag of nearly 10 billion dollars (8.8 billion euros).
Webb was built in the US and transported to its launch site in Kourou in French Guyana this year, where its launch date of December 18 was already pushed back after an incident.
This time a communication problem is to blame.
"The James Webb Space Telescope team is working a communication issue between the observatory and the launch vehicle system," a brief update on the NASA website reads.
"This will delay the launch date to no earlier than Friday, Dec. 24."
European Space Agency officials told AFP the problem was discovered after the telescope was installed on the Ariane 5 rocket on Saturday.
All the tests had to be run again to make sure everything was still working.
The Webb telescope, named after a former director of the American space agency, will be the largest and most powerful telescope ever to be launched into space.
It follows in the footsteps of the legendary Hubble but will be located much further from the sun. It is hoped that it will show humans what the Universe looked like even closer to its birth nearly 14 billion years ago.