Europe's top 32 football clubs have had 6.1bn euros (£5.27bn) wiped off their value since February 2020, says KPMG.
The fall ends five years of growth and is largely attributed to a loss of broadcast and matchday income because of the coronavirus pandemic.
Real Madrid top the study which looks at profitability, broadcasting rights, popularity, sporting potential and stadium value.
Barcelona leapfrogged Manchester United into second place.
Business services group KPMG found that the Old Trafford club's "enterprise value" fell by 20% compared with 2020, seeing them slip to third in a table they topped in 2017 and 2018.