United, who play at Real's Bernabeu stadium in Wednesday's Champions League last-16 first leg, topped Deloitte's Money League for eight years between 1997 and 2004. Then the Spanish champions surged past and they have led the way in annual income ever since.
Real became the first club to surpass the 500-million-euro ($669-million) threshold last season, according to Deloitte's latest rankings published last month.
Their earnings of 513 million euros beat La Liga rivals Barcelona into second place on 483 million, with United a distant third on 396 million.
"The Spanish club's revenue growth has been remarkable," Deloitte wrote.
Real have used a large chunk of those extra earnings to strengthen their squad, including buying Portuguese forward Cristiano Ronaldo from United in 2009 for a record fee of 94 million euros.
According to the latest transfer review by Prime Time Sport, Real have spent 428 million euros on players over the past four seasons, more than double United's 193 million.
While United have failed to keep pace with Real in revenue growth in recent years, they did seal a record $559-million deal with General Motors for Chevrolet to become their shirt sponsor for seven years beginning in 2014/15.
That, and success on the pitch in domestic and continental competition, could see them challenging Real and Barca, who earn far more than United from television rights, for the top spots again in the future, Deloitte said. ($1 = 0.7474 euros)