The project is connected with the contentious liquefied gas terminal in Aquilinia near the border with Slovenia. The decision to file an administrative lawsuit was taken by the government in October, the Environment and Spatial Planning Ministry said in a press release on Monday.

Slovenia has joined in the lawsuits the Italian Municipalities of Trieste and Muggia, the government of the Friuli Venezia-Guilia region and the Slovenian border municipality of Ankaran.

The lawsuits have been filed against the Ministry of the Environment, Conservation and the Seas and the Ministry of Cultural Assets and Tourism at the Roma-based Lazio Administrative Court.

The Slovenian ministry said that the environmental approval for the pipeline was an important decision in the procedure to acquire a building permit, which is yet to be issued by the relevant Italian authorities.

The ministry noted that the gas pipeline project had originally been a part of the Aquilinia gas terminal project, only to be later separated in the environmental impact assessment (EIA) procedure.

Slovenia thinks that the gas terminal is not in line with the Trieste port urban plan, and that the environmental approvals run against the EIA directive and do not provide for a favourable situation in the Gulf of Trieste, it added.

The ministry noted that Slovenia was constantly expressing its opposition to the Aquilinia project and the connected Trieste-Grado-Vilesse pipeline project at meetings with Italian representatives.

It added that environmental approvals for the terminal and pipeline did not mean by any stretch that the internal procedures in Italy for their realisation had been finalised.

"We are convinced that the Italian government will assess in further procedures whether the terminal is actually a strategic energy project, and take adequate decisions on this basis."