Gazprom and its European partners - E.ON, Wintershall, Shell, OMV and Engie - agreed the Nord Stream 2 project, which would pump Russian gas to Germany, last year.
In December 2015 they sent a filing to UOKiK requesting clearance of the joint venture.
"The office found that the concentration might lead to restriction of competition," UOKiK's statement said.
"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," it added.
UOKiK's
