Bulgaria's government has signed with Russian state-owned energy giant Gazprom the final investment decision for the construction of the South Stream gas transit pipeline.

The South Stream agreement was signed in the Bulgarian capital Sofia shortly before noon on Thursday by Bulgarian Prime Minister Boyko Borisov and Gazprom CEO Alexei Miller, in addition to another agreement for the Russian natural gas supplies for Bulgaria, which stipulates a 20% discount as of January 1, 2013.

The South Stream pipeline is intended to transport up to 63 billion cubic meters of natural gas to central and southern Europe, diversifying Russian gas routes away from transit countries such as Ukraine. Construction will start in December 2012, and not 2013 as previously planned.

The pipes will go from Russia to Bulgaria via the Black Sea; in Bulgaria it will split in two – with the northern leg going through Serbia, Croatia, Hungary, and Slovenia to Austria and Northern Italy, and the southern leg going through Greece to Southern Italy. Recent reports have indicated, however, that Russian energy giant Gazprom may give up on the construction of the offshore section of the South Stream gas pipeline to Austria.

The Black Sea underwater section of South Stream between Russia and Bulgaria will be 900 km long, and will be constructed at a maximum depth of 2 km.

In order to service the supplies for South Stream, Russia will expand its own gas transit network by building additional 2 446 km of pipelines with 10 compressor stations with a total capacity of 1473 MW, a project to be called "South Corridor" and to be completed in two stages by 2019.

The construction of the South Stream gas pipeline will begin in December 2012, and the first supplies for Europe are scheduled for December 2015.

The pipeline's core shareholders include Gazprom with 50%, Italy's Eni with 20% and Germany's Wintershall Holding and France's EDF with 15% each.

Gazprom has already established national joint ventures with companies from Austria, Bulgaria, Croatia, Slovenia, Greece, Hungary and Serbia to manage the onshore section of the South Stream pipeline.

Bulgaria recently committed itself to speeding up the construction of the Russian-sponsored pipeline on its territory, since on January 1, 2013, the EU is introducing new requirements for the access to energy networks.