WASHINGTON, Nov 16 (Reuters) - U.S. business inventory accumulation increased solidly in September, but motor vehicle retailers continued to struggle to restock amid an ongoing global semiconductor shortage, which has undercut production. Business inventories rose 0.7% after rising 0.8% in August, the Commerce Department said on Tuesday. Inventories are a key component of gross domestic product. September’s increase was in line with economists’ expectations. Inventories increased 7.5% on a year-on-year basis in September. Retail inventories slipped 0.2% in September as estimated in an advance report published last month. That followed a 0.2% gain in August. Motor vehicle inventories slumped 2.9% instead of 2.4% as estimated last month. Retail inventories excluding autos, which go into the calculation of GDP, increased 0.7%, instead of 0.6% as estimated last month. A slower pace of inventory decline in the third quarter accounted for all of the 2.0% annualized increase in GDP growth last quarter. COVID-19 pandemic-related shortages are making it difficult for businesses to rebuild depleted stocks. Restocking is expected to underpin manufacturing and economic growth in the fourth quarter. Wholesale inventories rose 1.4% in September. Stocks at manufacturers gained 0.8%. Business sales surged 0.9% in September after edging up 0.1% in August. At September’s sales pace, it would take 1.26 months for businesses to clear shelves, unchanged from August. (Reporting by Lucia Mutikani; Editing by Andrea Ricci)