NASA on Tuesday said its James Webb Space Telescope will miss its planned 2019 launch window. The setback, not exactly unexpected, means the next-generation telescope won't leave Earth until approximately May 2020 at the earliest.

The telescope, described by acting NASA administrator Robert Lightfoot as the highest priority project for the agency's Science Mission Directorate and the largest international space science project in US history, was developed in coordination with the Canadian Space Agency and the European Space Agency.

Work on the JWST started in the mid-90s as the Next Generation Space Telescope (the observatory was renamed in September 2002 after former NASA administrator James Webb). The project has been plagued by budget overruns and delays spanning multiple decades. Construction was finally completed in late 2016 with a planned launch in October 2018 but again, delays got in the way

Lightfoot on Tuesday said all of the telescope's flight hardware is now complete but added that issues brought to light with the spacecraft elements are forcing them to delay launch.

Specifically, NASA said testing of the telescope element and spacecraft element demonstrated that each system individually meets their requirements. A review board recently determined, however, that more time is needed to test and integrate the components together. Once that is done, environmental testing will be conducted at Northrop Grumman Aerospace Systems in Redondo Beach, California.