Airbnb has been fined $30,000 (£24,000) by the government of Catalonia over what it calls a "serious" breach of local laws.
The website, founded in 2008, now boasts more than half a million private properties and is valued at $10 billion. Catalonia, and Barcelona in particular, has been a key factor in this growth, often ranked as one of the site's largest markets. Last April Airbnb even launched a Catalan version of the site.
The fine was enforced for breaching local laws that state any flat rented to tourists must be registered with the Tourism Registry of Catalonia. Local regulations also forbid the renting out of rooms in private residences.
While Airbnb was just one of eight letting websites fined by the Catalan government, its success has seen it become the focal point for groups opposed to private tourist lets.
Critics of the site say that it drives up house prices in central districts and provides unfair competition to hoteliers.
Reme Gómez, of the Barri Gòtic neighbours association in Barcelona's Gothic quarter also claimed that the ever-changing roster of tourists had a negative effect on the community spirit in these areas. "People are making money and the rest of us are paying for it," she said.
Her organisation has spent the last decade campaigning for a ban on private tourist lets, stating that tourists are not always respectful of the locals.
"Tourist flats oblige the rest of us to live in a hotel, but without any of the same conditions," she said. "If you're in a hotel and it's 02:00 and the other guests are being too loud or vomiting in the stairwell, you can call someone to deal with it. Here no."
However, many have stressed the positive impact Airbnb is having in these areas. A recent study by the site found that 75 per cent of the hosts in Barcelona earn equal or less than the average income for Catalonia, with the additional money from the site helping them to keep their homes.
This week, Airbnb expressed its disappointment over the Catalan government's decision. "Barcelona should stay on the cutting edge of innovation," the company said in a statement, adding that the government's decision to issue a fine "will hold the city back."