Real Madrid and Barcelona have hit out at a European Commission (EC) ruling that they must repay millions of Euros worth of "illegal state aid".

The Spanish giants are among seven professional clubs who have ordered to reimburse the state for the funds, which it is claimed gave them an unfair advantage, breaching European Union state aid regulations.

Madrid face repayments of up to €23.4million in relation to alleged tax privileges and the overvaluation of land, but the club have vowed to appeal the ruling, questioning the methods in calculating the sum.

"It is a surprise that the European Commission have used a valuation made by an architect's office in Barcelona to dictate their decision, when said firm has little experience in making similar estimates in general and almost none in the city of Madrid," read a statement.

"Real Madrid, despite the only objective and legally obliging valuation being the cadastral value, presented in the report a valuation made by one of the most important companies in the world, Aguirre Newman, whose report concluded that Real Madrid had even been under-compensated by more than 7.5 million euros."

Barca have been asked to repay up to €5m over their alleged tax privileges and said: "Since January of 2016, FC Barcelona is taxed according to the reformed corporate tax code and no longer has any difference in taxation with respect to other Spanish clubs.

"The club will defend the interests of its members against abusive interpretations of the law."

Valencia – another one of the seven clubs which also include Athletic Bilbao, Osasuna, Hercules and Elche – claim they have not been officially notified of the decision.