The European Commission has begun an investigation after Italian officials spent £613,000 on a gig by Sir Elton John.
The performance took place at the Piedigrotta festival in Naples last September, but details of the cost have only just emerged.
Under EU rules, Commission money can be used to co-finance cultural projects.
But regional policy spokesman Ton van Lierop said the Commission was unsure how the Campania regional authority justified spending the money on a concert.
He told BBC News: “We want to know why this concert benefited this region.”
Dario Scalabrini, who helped organise the festival, said the event have helped promote Naples and strengthen local business.