United Airlines said on Tuesday that it planned to begin flying Boeing's 787 Dreamliner five days sooner than previously expected, a sign of confidence in the jet's return to service after regulators grounded it in January over safety concerns.

United, a unit of United Continental Holdings Inc , said it planned to use Boeing's newest jet on its Denver-Houston route starting May 31 and on the Denver-Tokyo's Narita route beginning on June 10.

The airline will make additional schedule changes as the timeline for certification and modification work becomes clearer, spokeswoman Christen David said.

David could not provide further details or say why the airline moved up its scheduled use of the jet. She also did not say would happen if the jet is not ready for use by May 31.

In February, United dropped the Dreamliner from its schedule through June 5, except for a Denver-Tokyo route tentatively scheduled for May. Many of the eight other airlines that own the jet also dropped it from their schedules.

Regulators banned the Dreamliner from flight on January 16 after lithium-ion batteries overheated on two jets that month. The jet has not flown commercial service since.

The U.S. Federal Aviation Administration gave Boeing permission for a single "ferry" flight on February 7 and on March 12 allowed the company to conduct a body of tests designed to show its redesigned battery system was safe.

On Friday, Boeing made a test flight with the modified system, completing initial testing of it.

The FAA said on Monday that it expected formal data from Boeing's tests to arrive shortly. Boeing declined to say whether it had submitted the data.

"While testing is completed, we are engaged with the FAA to reply to additional requests and continue dialog to ensure we have met all of their expectations," Boeing spokesman Marc Birtel said.

It was not clear if other airlines are scheduling 787 service.

LOT Polish Airways said it would not fly the jet until October. Japan's All Nippon Airways <9202.T>, the largest 787 owner, said late last month that it also was taking reservations for flights on the jet for June, but might decide to use different planes.

Shares of United Continental were up 0.1 percent at $30.10 in midday trading, while Boeing fell 0.4 percent to $86.40.

(Reporting by Nivedita Bhattacharjee in Chicago; Editing by Alwyn Scott and Lisa Von Ahn)