Virgin Galactic does not plan on sending paying customers to space until the last quarter of 2022.
"Recent material testing returned new data that requires further analysis," the company said in a statement.
CNN reports that the company had planned to begin commercial space flights in the third quarter of 2022.
Richard Branson, the founder of Virgin Galactic, successfully reached space in the VSS Unity in July.
The VSS Unity and VMS Eve will receive enhancements and modifications prior to the next test flight, according to the company.
"The enhancement program is designed to further increase margins that will enable improved reliability, durability and reduced maintenance requirements when in commercial service," the company said.
The test flight will include two members of the Italian Air Force, an aerospace engineer and a Virgin Galactic employee, Bloomberg reported.