Virgin Galactic Holdings (SPCE -5.11%) will resume test flights as soon as May 22, a huge step in the space tourism start-up's effort to launch service before the end of 2021.

Virgin Galactic had originally hoped to start commercial flights last year, but a combination of pandemic-related delays and some testing setbacks has it behind schedule. The company last week revealed a new potential complication involving the Eve, the plane that is used to get its spacecraft airborne, which could have potentially led to significant further delays.

Shot from below of the Eve carrying the Unity to altitude.

Virgin Galactic's Eve with its Unity spacecraft below. Image source: Virgin Galactic.

On Thursday, Virgin Galactic gave the all clear for the Eve to fly.

"Following a detailed inspection and thorough analysis of our mothership, Eve, we have cleared our Spaceflight System for our upcoming flight," CEO Michael Colglazier said in a statement.

The next test flight is tentatively scheduled for May 22, pending weather and technical checks. The company plans for the flight, which will be crewed by two pilots, to carry research payloads for NASA. If so, Virgin Galactic will generate revenue during the current quarter.

If the test flight goes well, it will go a long way toward making up for lost time last year. The test will verify the remedial work done in response to issues with a Dec. 12 test flight, as well as incorporate all of the objectives that were part of the original testing schedule.