Mercury emissions regulations are expected to be tightened in the US as the Senate clears the way for a vote repealing an existing EPA ruling on the issue. A Democrat-backed move in the Senate aims to repeal an Environmental Protection Agency regulation issued in March giving utilities until 2018 to cut mercury emissions by around two thirds current levels.

The more than 1,000 coal-fired power stations in the US are the largest unregulated source in the country, emitting some 48 tonnes of mercury annually.

The EPA rule, which has been heavily criticised by environmental groups as being too lenient, would limit emissions to 38 tonnes by 2010 and 15 tonnes by 2018 with the framework of a cap and trade mechanism.

In a procedural move, the Senate agreed to allow a vote on repealing the regulation, which is expected in the next few days, in preparation for introducing stricter standards.

The White House is expected to veto any attempt to overturn the ruling on the grounds that it would “compromise incentives for the power sector to invest in the development of reliable and cost-effective mercury control technologies.” Concerns have also been raised that a tougher mercury standard will force utilities to switch to more expensive natural gas, raising electricity prices.