GUE/NGL group president Gabi Zimmer has raised a number of "basic issues" of concern to her group with regards to plans for an energy union.
She urged the European commission and council to "approach security of supply with citizens' needs in mind - and not those of corporations".
The GUE/NGL president claimed that, "While the council says it wants the EU to be both number one when it comes to renewables and less import-dependent, what it puts forward does not go in this direction."
"As far as we are concerned, fracking is not appropriate and will lead to even more climate change" - Gabi Zimmer MEP
The German deputy also linked plans for an energy union to the ongoing transatlantic trade and investment partnership (TTIP) negotiations.
She identified fracking as the issue that links energy union to the controversial free trade agreement.
Zimmer noted that, "Fracking is one of the topics that will serve as a basis for discussions and that is why [the commission and the council] want to accelerate TTIP."
"As far as we are concerned, fracking is not appropriate and will lead to even more climate change. We want something different."
Neoklis Sylikiotis, a GUE/NGL vice-president added a warning that the energy union proposals do not address the growing problem of energy poverty.
The Cypriot MEP added, "We need to look at energy agreements between the EU and third countries. We need to adopt specific measures so that certain member states can solve their energy supply problems."
Sylikiotis also said any plan for an energy union package needed to be support efforts to bring an end to austerity.
Parliament's largest party groupings are broadly favourable to the commission's energy union plans unveiled last week. However GUE/NGL and the Greens/EFA group have raised a number of concerns.
The commission has said energy union will make "energy more secure, affordable and sustainable".