One-quarter of all transfer spending last summer took place after the start of the buying club's domestic season, a study from UEFA has found.
UEFA's annual "European Club Footballing Landscape" found that transfer spending in Europe last summer reached a new record of nearly €5.6 billion -- €1.5bn of that was spent by the top 10 leagues in the period in which the season and transfer window overlapped.
In September, the Premier League voted to end the summer transfer window before the start of the 2018-19 season onwards, which appears likely to have a massive affect on future dealings, according to data from last summer.
At €552 million, Premier League clubs spent easily the most amount of money in the 20-day overlap period, while France was second at €306m in 28 days.
La Liga was next at €236m in a 14-day period, ahead of the Bundesliga's €122m in 13 days. The English Championship's €89m in 28 days ranked fourth, outpacing even Serie A's €79m in 12 days.
In other transfer-related findings in the report, UEFA said summer transfer fees were 40 percent higher that the previous record a year ago.
The big five European leagues accounted for 80 percent of all transfer spending, an increase from 63 percent in 2010. Only four of the 96 players bought for at least €15m did not move to a team in those legues -- Zenit's trio of Leandro Paredes, Emanuel Mammana and Sebastian Driussi, and Porto's Oliver Torres.
The Premier League was the biggest spender at €1.68bn, but though its clubs also brought in the most money at €911m, the net spend was also the largest at €772m.
Thanks to its €222m purchase of Neymar, Paris Saint-Germain paid the most with a total of €418m, ahead of Manchester City's €249m.
Barcelona's sale of Neymar allowed them to make a €34m profit despite spending €193m, but that was nothing compared to Monaco's €256m profit from the sale of Benjamin Mendy and Bernardo Silva, among others.
UEFA also found that agent commissions averaged between 12.6 percent, though in 32 percent of deals studied the agent claimed a fee of more than 20 percent, mostly on deals worth less than €1m.