FARNBOROUGH, England (Reuters) - Boeing ( BA ) is set to announce on Monday an order for roughly 20 777X jets from Korean Air, in a boost to the U.S. planemaker's long-delayed plane programme, two sources familiar with the matter said.

Reuters first reported the deal earlier in July. Monday's announcement at the Farnborough Airshow could also include 787 jets, one of the sources said.

Boeing ( BA ) declined to comment.

South Korea's largest carrier has been in talks over a potential return to its traditional supplier of long-haul aircraft for months after placing a surprise order for 33 A350 jets from Airbus in March.

The 777X jets are worth some $198 million each after typical discounts, according to delivery prices from Cirium Ascend.

The 777X is the industry's largest twin-engined jet with around 400 seats but its entry into service has been delayed by five years due to problems including certification delays.

Boeing ( BA ) started certification flight testing of the 777-9 earlier this month with U.S. aviation regulators onboard.

The orders are set to be announced at a time when Boeing ( BA ) is wrestling with a quality crisis that has brought it under heightened regulatory and legal scrutiny after a door panel blew off near new 737 MAX 9 in January.

Boeing ( BA ) announced a separate order with National Airlines on Monday for four 777 freighters.