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Photographer: Jean Catuffe/Getty Images Photographer: Jean Catuffe/Getty Images

The seeding in the opening group stages of European soccer’s $1.1 billion Champions League tournament has been flawed for 15 years, giving an unfair advantage to clubs including Arsenal, according to a study by the U.K.’s Sheffield Hallam University.

European soccer’s ruling body UEFA based rankings on performance in the competition over the previous five seasons, which apparently has helped higher seeds to “monopolize” dominant positions, according to the study by Daniel Plumley and Stuart Flint.

UEFA’s seeding is a disadvantage to teams including Manchester City, the authors said, and they suggested it would be fairer to base the rankings on more recent performance. UEFA has tried to address the imbalance before Thursday’s group-stage draw by making the defending champion and seven national champions the top seeds, Flint said in an e-mail.

“There is still unfairness throughout the rest of the seeding,” Flint said.

For the random draw, the 32 teams in the group stage are divided into four “pots” which depend on their UEFA ranking. The eight top seeds can’t face each other. The ranking is based on the sum of points a team has won over five years and the perceived strength of its national league.

The authors, citing the Herfindahl-Hirschman Index, said there was gradually more competitive imbalance in the 15 years through the 2013/4 season. The index is a statistical analysis used to evaluate market share and decide whether mergers would be anti-competitive. UEFA distributed 940.6 million euros ($1.1 billion) of prize money in the last season of the study, the authors said.

The “flawed” system meant that Arsenal remained among the top seeds despite not finishing higher than third in the English Premier League for a decade, while Manchester City hasn’t managed to break into the higher seeding places even though it won the domestic league in 2012 and 2014, the authors said.

“Newly qualified clubs are immediately hindered by their low seeding,” the authors wrote.

The top seeds have finished in the top two of their groups in 86 percent of cases since 1999, according to UEFA data. Twelve of the last 16 winners started as top seeds in the season they won, with only Jose Mourinho’s Porto and Inter Milan, Rafael Benitez’s Liverpool and Carlo Ancelotti’s AC Milan bucking the trend, UEFA said.

UEFA’s rule change means record 10-time champion Real Madrid and Arsenal, which didn’t win their domestic leagues last season, are among teams to lose their position as top seeds. A UEFA official said the organization couldn’t immediately comment because it hasn’t reviewed the study.

In tennis seeding, three of the four Grand Slams follow player world rankings, which are compiled from tournaments over 12 months. Wimbledon, the only slam on grass, decides the men’s draw partly on performance on the surface the previous two years.

(Adds how tennis decides seeds in 11th paragraph.)