That strength is derived mainly from their stranglehold over about 600 million euros in annual revenue from audiovisual rights, which, in turn, is due to Spain's system of clubs negotiating with TV companies individually rather than collectively as in most rival European leagues.
Real and Barca, who meet at the Catalan team's Nou Camp stadium for a match that will be watched by millions around the world, suck in about half the total, which helps them pay the astronomical transfer fees and wages for the best players.
An agreement brokered this month with 11 other top-flight sides on sharing some of the TV cash from 2015 is likely to cement Real's and Barca's advantage while helping the others only marginally, analysts said.
Angel Barajas, associate professor of financial management at the University of Vigo, noted that conditions in the crisis-hit TV market may have changed significantly by 2015 and the clubs' hopes of reaping an extra 200 or 300 million euros may be optimistic.
"Given the lack of competitive balance it could be that La Liga is not sufficiently attractive to promote a bidding war for the rights," Barajas told Reuters.
"If things carry on the way they are, interest in La Liga will diminish and, as a result, income from TV."
"In the last five years the percentage of league titles going to the big two was 100 and in future years there will be no chance for any of the other clubs to aspire to competing for the title with Real Madrid or Barcelona," Sevilla president Jose Maria del Nido said last week.
"This is something that just does not happen in our European neighbours," he added. The six clubs have also said they will take the case to Spain's competition authority.
