Bulgaria's parliament relaxed on Thursday a ban on a controversial natural gas drilling method that had threatened to completely halt exploration in the country.

A total of 92 lawmakers of the 110 present in the 240-seat parliament approved the changes that open the way for the use of hydraulic fracturing or 'fracking' in test drilling for natural gas and oil.

But they left standing a ban on using the controversial method for production, specifically for shale gas.

Parliament had imposed in January a total ban on fracking -- a drilling technique that uses high pressure injections of water, sand and chemicals to blast through rock and release oil and gas trapped inside.

The hastily adopted regulations whose primary goal was to outlaw shale gas drilling had however blocked all exploration, even for conventional production, as the process is used at much lower pressure in test drilling.

This prompted protests from energy companies and triggered the changes adopted Thursday.

Bulgaria revoked in January a five-year test drilling permit granted last year to US oil giant Chevron, citing lack of sufficient assurances on the environmental safety of fracking.

Although popular in the United States, opponents warn of the method's environmental impact, insisting there is high risk of contaminating the land and drinking water and of triggering earthquakes.