In a speech in Brussels Energy Commission Günther Oettinger said that the Commission had decided to "launch infringement proceedings" against 17 national governments in the field of electricity and 18 national governments in the field of gas.
"I would like to call upon those Member States to take their responsibility and make all efforts to fully implement the [new rules]," he said.
The EU's third energy liberalisation package is part of a decade-long project intended to create a single European internal energy market. Member states had until 3 March to adopt two Directives, for gas and electricity, into their national laws.
Draft regulations implementing the directive are currently in front of Parliament for approval. A spokesman for the Department of Energy and Climate Change said the regulations should be implemented by early next month.
"We are in the process of transposing the EU Third Energy Package into law and should complete this by early November. We will be in touch with the European Commission shortly to update them on our progress," he said.
In the speech Oettinger stressed the importanance of the internal market.
"Gas and electricity would not flow freely across borders. Energy companies would not be able to take full advantage of economies of scale. The level of competition would be much lower, with less choice for customers," he said.
The third energy liberalisation package was adopted in July 2009. It aims to prevent the formation of energy company monopolies by splitting their energy production and supply activities, as well as making it easier for third parties to gain access to private pipelines.
The European Commission can take legal action against member states that do not correctly transpose EU law or fail to notify that they have passed national measures to implement EU rules.
The other countries are Austria, Belgium, Bulgaria, Cyprus, Denmark (gas only), Estonia, Finland, France, Ireland, Lithuania, Luxembourg, the Netherlands, Romania, Slovakia, Spain and Sweden.
Countries that do not comply with the requirements of EU law can be referred to the European courts.