Following a 0.2 percent gain in April and a 0.4 percent gain in May, the index dropped 0.2 percent in June to reach 104.8.
The index assigns a base value of 100 to the year 2004 to provide a reference point.
In June, four out of seven component indexes made gains in the month, including gains in equity markets and business and consumer confidence.
Despite the overall decline, " improvements in equity markets, business, and consumer sentiment in July suggest that the U.K. economic recovery is still ongoing," said Conference Board economist Brian Schaitkin in a statement.
"However, declines in income and employment so far this year demonstrate that labor markets may under-perform relative to output over the short term after a long period of flat to negative productivity growth," Schaitkin said.