The long narrow acute sepals suggest subsection Rostratae, rather than subsect. Viola for instance.
But the species is not obvious to me.
Lets try elimination: Not: elatior / persicifolia / canina / riviniana / reichenbachiana / lactea / rupestris / jaubertiana / mirabilis / pseudomirabilis / tanaitica / mauritii / sieheana / accrescens / jordanii / pumila.
That leaves - I don't know!
The elongate leaves suggest V. elatior/pumila, but the flowers are not right.
My Flora of Spain is not accessible at the moment, so I'm flummoxed for now. I'm sorry not to be able to answer your question, especially considering the fact that you went to the trouble of getting three really quite good photos.

According to a very complete guide of the region the Viola species present here are: cornuta, bubanii, arvensis, kitaibeliana, biflora, palustris, alba, pyrenaica, hirta, odorata, suavis, rupestris, riviniana, reichenbachiana, lactea, canina. None of them matches, but could it be a blue variety (or not so white) of V. lactea?? I'll try to herborize it and take pictures to classify.
Thanks

herborize ?!
There's a word you don't see every day!
Must remember to use it; thanks for the prompt.

A blue form of V. lactea is unknown to me, but there obviously has to be some answer to your question. The shape of the flower actually reminded me of a flower I had on V. jaubertiana, but it wasn't really typical. I wonder if your flower is typical.

Is that Flora Iberica you're looking at?

Hello folks

I have forwarwed the picture to a french violet botanist friend of mine and he wrote me - " that's certainely a Viola canina"
Hope it helps!!

Well, I've been watching my V. canina plants at home, and none of them have narrow sepals and a short fat spur like in the mystery-violet photo. I have considered the various varieties of V. canina discussed by Miss Gregory (1912), such as V. canina var. montana, but they are all described as having broader sepals like my plants. The narrow sepals are more like V. lactea, but the shape and colour of the petals is more like V. canina. Hybrids of these two species are known, but are rare, in the UK at any rate (Moore, 1998).

I note that Moore (1975, 1998) does not include sepal shape nor spur size/shape in his table for differentiating these two species, so maybe those features are more variable than I have observed (I have seen these species in only a small part of their range in Europe). On that basis, I concur with your friend, Natha, and would tentatively consider the plant to be V. canina.

Thanks to all. I was using a book called "Claves Ilustradas de la Flora del País Vasco y territorios limítrofes". I don´t know if herborize is a correct word in english, since it's not my maternal language :).

Ibaih,
'Herborize' is indeed a correct English word - "To search for plants, or new species of plants, with a view to classifying them.", to quote from Webster. The Oxford English Dictionary has, less interestingly: "To gather herbs; to botanize." and gives the first reference as far back as 1664. Isn't 1664 the name of a certain beer? How do you make beer? ...You start by gathering herbs...