The European Commission has asked the Bulgarian government to suspend work on Gazprom's South Stream gas pipeline pending a decision on whether it conforms with EU law.
A spokeswoman for the EC confirmed that the Bulgarian government had been sent a letter of formal notice about the 934-kilometre, four-line project from Russia to western Europe, Reuters reported from Brussels.
"Whilst discussions with the Bulgarian authorities are taking place and until there is full compliance with EU law, we have also asked the Bulgarian authorities to suspend the project," Commission spokeswoman Chantal Hughes said.
The notice letter, which must be responded to within a month, represents a preliminary step in a process that could eventually lead to full infringement proceedings and possible fines, she added.
It comes after the EC published an updated energy security strategy last month that called for the suspension of the project both over concerns about the tendering process and due to changing energy security priorities.
Much of the new strategy focuses on how the EU can diversify its energy supplies away from Russia in the wake of Moscow's annexation of Crimea, particularly in the eastern member states that are most dependent on Russian imports.
The EC also sees bilateral deals between Russia and the pipeline’s proposed calling points - Bulgaria, Serbia, Hungary, Greece, Slovenia, Croatia and Austria - as illegal because they prohibit non-discriminatory access to third-party gas suppliers.
As Upstream reported from Moscow last week, South Stream has shown no signs of slowing down, with plans to award contracts for the remaining two of four lines late this year and early next year.
Italy’s Saipem is already contracted to build one section of the pipeline from Russia to Bulgaria, while Swiss-registered Allseas is building another of the parallel lines in the Black Sea.
Already heavily dependent on Russian gas, Bulgaria wants to shore up its gas supplies with the new pipeline, while Moscow prioritises the project as a way to get gas to western Europe without going through Ukraine.