An expansion to a Russian natural gas pipeline in the Baltic Sea, Nord Stream, would inhibit market competition, a Polish antitrust authority said Friday.

The Polish antitrust authority, known by its acronym UOKiK, said it vetted plans by Russian energy company Gazprom to expand the Nord Stream natural gas pipeline against interests from six companies working in the regional energy sector, as well as the Polish Energy Regulatory Office.

"The office found that the concentration might lead to restriction of competition," the Polish authority said in a statement. "At the moment Gazprom has a dominant position with respect to transmission of gas to Poland, and the planned transaction could further strengthen the company's negotiating position with regard to users in Poland."

Moody's Investors Service said in a report this week from London that utility companies in Poland are less diverse than their European counterparts, though their credit profiles remain in good shape. With few tapped reserves of its own, Poland imports most of its oil and natural gas from Russia.

Gazprom signed shareholder agreements on the development of the second phase of the twin Nord Stream pipeline system last year with its counterparts at German energy companies BASF and E.ON, as well as those from French company ENGIE, Austria's OMV and Royal Dutch Shell.

Under the proposed expansion, two more lines would be added to the existing network, roughly doubling the pipeline's net capacity.

The European Union has expressed concern about Russia's control over the regional market as the Russian gas company typically controls both the transit networks and the reserves they deliver. Maros Sefcovic, a European leader on energy issues, said the Nord Stream expansion would "alter the landscape" of the regional energy sector by blocking new sources and suppliers in Europe.

Gazprom nevertheless the project would be successfully delivered and the gas pipeline would be commissioned before late 2019.