United’s earnings were £88.9m more than Manchester City (£538.2m), who are sixth in the rankings behind Bayern Munich and Paris Saint-Germain.
Liverpool (£533m) are seventh, while Tottenham (£459.3m) are the highest-ranked London club in eighth.
Chelsea (9th - £452.2m), Arsenal (11th - £392.7m), West Ham (18th - £185.8m) and Everton (19th - £182.8m) make up the remaining Premier League sides to make the top 20.
Marcus Rashford of Manchester United celebrates scoring their first goal during the Premier League match between Manchester United and Norwich City
Image credit: Getty Images
"Manchester United have been the top English club since we started the Money League but that could come under a bit of pressure when we do it in 2021," said Dan Jones, partner in the Sports Business Group at Deloitte.
"That will cover this season when Liverpool are having a tremendous season on the pitch, so we think there's a possibility of United losing that number one position.
"Part of that will ride on how the rest of this season goes, if City or Liverpool go deep into the Champions League.
"It is definitely a possibility that United will lose that place. Depending on where they are in that range of projected revenue (£560-580m), and if City and Liverpool do well in the Champions League, it could be very, very close.
"If you think about where we were 10 years ago, the idea that City would overhaul United in financial terms would have seemed far-fetched."
It is the first time Barca have topped the rankings, with the £73.6m gap to Real Madrid the biggest margin between first and second since Deloitte began publishing the list.
Jones added: "Barca are a clear example of a club adapting to changing market conditions, reducing the reliance on broadcast revenue and focusing on growing revenues within its control.
"The club's commercial operation generated 383.5m euros (£326.6m) of revenue, which is more than the total revenue of the 12th-placed club in this year's Money League.
"With the club expecting further growth of 30m euros (£25.5m) in commercial revenues and total revenue of almost 880m euros (£749.5m) in 2019-20, we expect them to retain the top spot in next year's edition."