Manchester United are at risk of being overtaken by Manchester City and Liverpool in Deloitte's Football Money League, according to the latest rankings.
United's earnings of £627.1m for 2018-19 puts them third overall in this year's list, behind Spanish giants Barcelona (£741.1m) and Real Madrid (£667.5m).
Their closest Premier League rivals are City and Liverpool in sixth and seventh respectively, with City just £88.9m behind.
Tottenham, meanwhile, are now the top-ranked London club in eighth - their highest ever position in the rankings, with Chelsea and Arsenal in ninth and eleventh respectively.
Manchester United have been the highest-ranked English club since Deloitte first compiled the Money League survey based on the 1996-97 financial results.
However, they are forecast reduced revenues of between £560-580m for 2019-20 with the prospect of a second consecutive season without Champions League broadcast revenue.
Dan Jones, partner in the Sports Business Group at Deloitte, believes a changing of the guard is possible.
"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," he said.
"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."
Deloitte recorded United's revenue in 2008-09 as £278.5m, with City almost £200m behind them on £87m. At that point United were almost £100m ahead of Liverpool as well.
In all, there are eight Premier League clubs in the Money League top 20, more than any other country, and all the clubs hail from Europe's big five leagues of England, France, Germany, Italy and Spain.
Tottenham have moved up from tenth to eighth in the rankings with revenue of £459.3m, while their north London rivals Arsenal have fallen from ninth to 11th after a second consecutive season without Champions League football.
Chelsea, who announced an after-tax loss of £96m for 2018-19 on New Year's Eve, have dropped one position to ninth in the table, while the other Premier League representatives are West Ham and Everton in 18th and 19th respectively.
Combined revenues among the top 20 have grown by 11 per cent compared to last year to a record of 9.3 billion euros (just under £7.9billion).
Barcelona lead bitter rivals Real by £73.6m, a record gap between first and second in the history of the Deloitte study.
Deloitte said a "primary factor" behind the Catalan club's earning power was their decision to bring merchandising and licensing activities in-house.
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."