Bulgaria must commit more thoroughly to the development of the EU-sponsored Southern Energy Corridor (also known as Southern Gas Corridor) aimed at diversifying natural gas suppliers to Europe, according to a leaked report of the European Commission.

The draft report of the EC, which is still to be released, criticizes Bulgaria for throwing its weight mostly behind the Russian-sponsored South Stream gas transit pipeline, while lacking sufficient commitment to EU's attempts to develop the Southern Energy Corridor, EurActiv reported Tuesday citing the leaked report.

Bulgaria needs to complete the ongoing investment projects on gas interconnectors with Romania, Serbia and Greece, and make reverse flows possible on its interconnector with Turkey, the EU executive says.

"Bulgaria also needs to play a more proactive part in opening up the Southern Gas Corridor, which has the potential to diversify supply sources," the paper reads.

The Southern Gas Corridor is a key element of competing projects to bring natural gas to Europe from the offshore Shah Deniz II field in Azerbaijan.

Up to now, Bulgaria has made commitments to South Stream, a Gazprom-favoured project widely seen as a competitor to the Southern Gas Corridor.

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.

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.

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 has 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.

In the draft gas supply report of the EC, Bulgaria is also urged to increase cross-border network capacity.

Regardless of its more favorable geographic location, Bulgaria is also singled out as one of the energy infrastructure black spots on the EU map.