WASHINGTON, Dec 11 (Reuters) - U.S. wholesale inventories rebounded in September amid increases in the stocks of long-lasting manufactured goods, which likely provided a lift to gross domestic product in the third quarter.

Stocks at wholesalers increased 0.5% after falling 0.1% in August, the Commerce Department's Census Bureau said on Thursday. Economists polled by Reuters had forecast inventories edging up 0.1% in September.

The report was delayed by the recently ended 43-day shutdown of the government. Inventories, a key part of gross domestic product, increased 1.8% on a year-over-year basis in September.

Wholesale stocks of durable goods rose 0.3%, driven by computer equipment, metals and electrical products. But motor vehicle inventories were unchanged.

Business inventories decreased at a $18.3 billion annualized rate in the second quarter, subtracting 3.44 percentage points from GDP. That was, however, more than offset by a record 4.83 percentage point contribution from a smaller trade deficit.

The Atlanta Federal Reserve is forecasting gross domestic product increased at a 3.5% annualized rate in the third quarter. The government will release its first estimate of third-quarter GDP on December 23 after it was delayed by the shutdown. 

The economy grew at a 3.8% pace in the April-June quarter. While the rebound in inventories likely added to GDP growth last quarter, it also reflected softening demand. 

Sales at wholesalers slipped 0.2% in September after declining by the same margin in August. At September's sales pace it would take 1.29 months to clear shelves, up from 1.28 months in August.