SEATTLE, Jan 21 (Reuters) - Discount airline Norwegian Air Shuttle ASA said on Tuesday that a fuel leak on a Boeing Co 787 Dreamliner caused a flight from Bangkok to Oslo to be delayed by 19 hours.

The aircraft was preparing for departure on Sunday with passengers on board when crew members noticed a fuel leak, a Norwegian spokesman told Reuters.

The leak was caused by a problem with a fuel valve, the spokesman said. Passengers were taken off the plane, which was taken out of service. The flight departed 19 hours later and the jet was later sent to a maintenance facility in Stockholm.

Norwegian said it did not have more detail about the problem because Boeing is responsible for maintenance of the airline's 787 fleet.

Boeing said it was working with Norwegian Air on the issue.

The incident is one of a number of problems that 787 operators have experienced with Boeing's newest jetliner.

Technical problems are not unusual when new aircraft enter service. Just over a year after the 787 began commercial operation in late 2011, the worldwide fleet was grounded in January 2013 after lithium-ion batteries overheated on two 787 jets in less than two weeks.

In October, Norwegian Chief Executive Bjorn Kjos praised the jet's flying performance and fuel efficiency, but expressed concern that the airline was forced to take one of the new $200 million jets out of service for two weeks and lease an alternative jet because of problems with a hydraulic pump.

Boeing redesigned the pump to solve the problem.

Last week, a battery melted and emitted smoke from a parked 787 Dreamliner operated by Japan Airlines. The incident at Tokyo Narita airport caused the jet to be taken out of service.

The fuel leak on the Norwegian jet in Bangkok, and other incidents, have combined to lower the plane's ability to consistently depart on time on scheduled flights. Most older aircraft are highly reliable, able to leave the gate on time more than 99 percent of the time.

In October, Boeing Chief Executive Jim McNerney said the company was "making good progress" on improving the reliability of the 787, but that he was not yet satisfied with the performance, which was about 97 percent on time. That means for every 100 flights, three are unable to leave on time due to mechanical issues with the plane.

Norwegian did not provide a reliability level, but spokesman Lasse Sandaker-Nielsen said that recently the "overall reliability of our 787 fleet has not been satisfactory."