The report also found that teams winning groups were finishing with more points than 15 years ago and teams coming bottom with less -- all pointing to an increase in the gap between the elite clubs and the rest.
"The different indicators analysed illustrate a clear trend towards less balance and more predictability," said the Swiss-based CIES (International Centre for Sports Studies) Football Observatory.
The study found that the number of "very unbalanced matches", where one team wins by three goals or more, had increased from 16.9 percent of group matches between 2003 and 2006 to 22.9 percent between 2015 and 2018.
During the same period, the average number of points won by the eventual group winners rose from 2.11 to 2.26 per game, while the goal difference of the group-winning teams rose from 6.38 after the six matches to 8.91.
Meanwhile, the average points-per-game of the teams finishing bottom of the groups fell from 0.59 to 0.45 and their final goal difference from -6.72 to -9.06.
"A thorough rethink is needed to preserve a sufficient level of sporting and economic balance both within the competition and in European football," the report said.
(Reporting by Brian Homewood, editing by Ed Osmond)
