WASHINGTON, Oct. 9 — Defense Secretary Robert M. Gates has approved a plan to ease the strain of two wars on the military by increasing the size of the active-duty Army to 547,000 by 2010, two years sooner than planned, officials said Tuesday.

Mr. Gates approved the accelerated timetable in a Sept. 26 memo that also barred the Army from reaching the goal by lowering its recruiting standards or employing “stop loss,” a practice of prohibiting soldiers from retiring. The memo put the plan’s cost at $2.6 billion over the next five years.

Gen. George W. Casey Jr., the Army chief of staff, announced the 2010 goal in a speech on Tuesday at the Association of the United States Army convention here. Army officials had previously said the timeline would be reduced by one year, to 2011.

Army officials said they could achieve the increase through retention and recruiting, but conceded that achieving the goal in three years, rather than five, was ambitious. “Meeting this target will not be easy,” General Casey said.