Aug 31 (Reuters) - U.S. single-family home prices in 20 key urban markets rose in June from a year earlier at the fastest pace on record, a closely watched survey said on Tuesday. The S&P CoreLogic Case-Shiller composite index of 20 metropolitan areas gained 19.1% through the 12 months ended in June, from an upwardly revised 17.1% the previous month, marking the largest annual price increase in the survey’s 20-year history. A Reuters poll of economists had forecast a 18.5% increase. On a month-to-month basis, the 20-city composite index rose 1.8% from May. This was in line with estimates from economists polled by Reuters. (Reporting by Evan Sully Editing by Chizu Nomiyama) Our Standards: The Thomson Reuters Trust Principles.