According to Consumer Law, yes you can.
As it’s not the main business of either to provide ‘hostelería’ services, and consumers should be free to choose where they buy food and drinks. Obliging those who have bought tickets to buy food and drinks on the premises, is considered ‘abusive’.
A ruling from the Supreme Court in Castilla-La Mancha re a movie cinema complex where they didn’t allow those that had bought tickets to bring their own food/drinks into the viewing room with them confirms:
“it is unreasonable to understand the limitation of the consumer’s choice capacity” and that “if you limit the possibility of accessing the room based on the origin of the products and you can only consume those purchased inside, it turns out that indirectly it is imposing that it uses services that in principle it has not requested but that is forced to order from the own company “.
Sadly, many Music Festivals do not appear to pay any attention to consumer law. Only this year, FACUA, one of the largest consumer organisations, has already denounced 25 music festivals and/or the organisations promoting and selling tickets for failing to comply. Several of them ‘repeat offenders’ and having been denounced last year as well.
FACUA asks to be informed of any music festivals or cinemas where the public is prohibited from taking in their own food and drinks.