Bulgaria's Prime Minister Boyko Borisov should immediately apologize for his threatening his fellow citizens he would sign a contract on shale gas production unless they stop "nagging him", left-wing politician Sergey Stanishev has demanded.

"Nothing will happen if you stop irritating me. About those holding the signs, I wish to tell that they are enticed by a particular political party. If this keeps going on, we will sign the contract just out of sheer stubbornness," Borisov said Sunday upon noticing citizens from the northeastern city of Dobrich protesting against the potential threat of environmentally dangerous shale gas exploration in the region.

"Borisov should get in touch with reality," Stanishev, leader of the Bulgarian Socialist Party and a former PM (2005-2009), commented Monday on his successor's controversial statement.

"He is chosen by the Bulgarian Parliament as Prime Minister, he has to serve the citizens. He is not a Sultan," Stanishev reminded. He pointed out that Bulgaria still was a European democracy, in which everyone was entitled to an opinion.

In June, the Bulgarian government gave US energy giant Chevron a permit to prospect for shale gas in a large section of Dobrudzha, in the north east of the country, where Dobrich is situated.

Environmentalists and local residents fear that the technique, which involves pumping unspecified chemicals into the ground at high pressure - so-called hydraulic fracturing - might pollute soils and groundwater in Dobrudzha, one of the most fertile regions in Bulgaria.

Shale gas extraction has been a controversial technique, after a number of wells in the US - the country that pioneered it - leaked, causing massive damage to drinking water.

The US government is currently holding a massive survey of the method and the need to regulate it, while in July France became the first country in the world to impose an all-out ban on it, after protests from local residents.