The decision had been expected after Boeing said on June 27 it would likely take until at least September to fix a fresh flaw in the grounded 737 MAX discovered by the Federal Aviation Administration last month.
United had previously left its 14 737 MAX jets off its flight schedule through early September.
The new potential risk must be addressed before the aircraft can return to service since two deadly crashes in Ethiopia and Indonesia led to its worldwide grounding in March.
Once regulators approve the MAX for flight, each aircraft will likely require between 100 and 150 hours of maintenance preparation before flying, in addition to new pilot training, airline officials have said.
Among other U.S. MAX operators, Southwest Airlines Co so far has canceled flights through Oct. 1 and American Airlines Group through Sept. 3, though those timelines are also likely to be extended.
While the three U.S. airlines' MAX jets account for only a fraction of their entire fleet, the grounding has forced about 150 daily flight cancellations for Southwest and 115 for American.
United is due on Tuesday to publish second-quarter results, which investors are hoping will provide details on the financial impact of the grounding.
Unit revenues, based on per available seat mile, could benefit as planes fly with more passengers. But the longer the grounding lasts, the more strain on airlines counting on fresh deliveries of the fuel-efficient MAX this year to renew and expand their fleets.
United, for example, was expecting a total of 30 MAX jets by year-end, meaning that its daily flight cancellations are increasing for every month that the aircraft is grounded.
Deliveries remain frozen until regulators approve the MAX's return to service.
Some analysts do not expect the jet to fly commercially before the end of the year.
"We'll continue to monitor the regulatory process and nimbly make the necessary adjustments to our operation and our schedule," United said in a statement.
The 737 MAX was grounded worldwide in March after an Ethiopian Airlines plane plunged to the ground soon after take-off, five months after a similar Lion Air fatal crash off the coast of Indonesia.
Boeing hopes a software upgrade and new pilot training will add layers of protection to prevent erroneous data from triggering a system called MCAS, which was activated in both the planes before they crashed.
(Reporting by Tracy Rucinski; Editing by Richard Chang)
By Tracy Rucinski