The index, which had stumbled in June, rose 0.7 percent in August to 105.4.
The index uses 2004 as a base year, assigning it the value of 100 to provide a reference point for comparisons.
Six out of seven component indexes made gains in August.
"The outlook for the U.K. continued to brighten in August, as the LEI increased markedly," said Conference Board economist Bert Colijn.
"The improvement among the indicators was widespread ...," he said.
Colijn said limited support from economies around the world could slow the U.K. growth.
"A substantial acceleration in economic growth in the months ahead seems unlikely as the global economic environment remains uncertain," he said.