The American economy grew last quarter at its fastest rate in over a decade, providing the strongest evidence to date that the recovery is finally gaining sustained power more than five years after it began.

Bolstered by robust spending among consumers and businesses alike, economic output rose at an annual rate of 5 percent during the summer months, the Commerce Department said Tuesday, a sharp revision from its earlier estimate of 3.9 percent. The advance followed a second quarter where growth reached a rate of 4.6 percent after a decline last winter that was exacerbated by particularly harsh weather.

The revision was led by an upswing in investment by businesses, a powerful force for growth in most economic recoveries but one that has lagged in the latest rebound. Higher consumer spending, including increased outlays on health care, and a narrower trade balance also contributed to the summer improvement. The gain makes the third quarter the strongest since the summer of 2003.