A key test flight for the Boeing 737 MAX, a major hurdle before the grounded plane is cleared to fly again, will not take place before June, sources told AFP on Thursday.

The coronavirus pandemic is partly to blame for the delay. Boeing had previously targeted mid-2020 to win approval from the US Federal Aviation Administration for the aircraft that has been out of service since March 2019 following two crashes that killed 346 people.

One source told AFP that no test flight would take place before June, while second source said it likely would be pushed beyond June and no date had been set.

European and Canadian regulators, who have been planning to join their US counterparts for the flight, have been unable to schedule a trip because of social distance policies, one of the sources said.

A spokesperson for the European Union Aviation Safety Agency said work was "progressing" on the MAX despite lockdowns.

"However, the impact of COVID-19, particularly the travel restrictions it has brought, mean that we do not have a firm timetable for the test flights, which are required to complete the validation for return to service," the EASA spokesperson said.

An FAA spokesman said there was no update on the timing of certification. The agency has said it will only move ahead when its safety concerns are met.

A Boeing spokesman said the company was continuing to work with the FAA ahead of a certification flight.

Boeing Chief Executive Dave Calhoun said in April that some of the documentation has taken longer than expected and that COVID-19 restrictions had had affected the process.