The Bulgarian parliament banned on Wednesday shale oil and gas exploration through hydraulic fracturing or fracking due to environmental concerns following widespread protests against the unconventional gas.

A total of 166 deputies in the 240-strong chamber voted to impose a permanent ban on fracking, an exploration method which involves injecting water mixed with sand and chemicals at high pressure into shale formations.

Deputies said there were not enough proofs the drilling method was environmentally safe. Critics worry it may poison underground waters, trigger earthquakes and pose serious hazards to public health.

On Tuesday, the government cancelled a shale gas exploration permit to U.S. energy major Chevron.