The European Commission has demanded that Budapest give its account about a legal amendment that Brussels believes is related to the construction of South Stream.
Hungary passed on Monday changes to energy legislation allowing companies to build pipelines without having a license as system transmission operators and, virtually, without consent from the Commission.
"We understand that this text [has been] adopted over South Stream," Anna-Kaisa Itkonen, EU Commission Spokesperson for Climate Action and Energy is quoted by the Bulgarian National Radio as saying.
Other officials said Tuesday the Commission had already communicated to Hungary to seek an explanation.
In March Bulgaria also approved changes to its existing legislation which technically enables Sofia to go around European law. This prompted a reaction from the EU Commission, which later demanded that construction activities be halted. The project was subsequently frozen.