DRAX power station has been given the go-ahead to start commercial burning of a controversial fuel after two years of tests showed "no detectable impact" on the environment.

But district councillors, sceptical about the long-term effects of burning petcoke, have pledged to continue monitoring pollution levels in Selby.

Selby District Council decided to keep on monitoring for at least another year.

A spokeswoman for Drax said the company was happy to continue testing.

Petcoke, or petroleum coke, has been branded a "fuel from hell" by environmental campaigners.

It is a heavy hydrocarbon, and a byproduct of the US oil industry.

The fuel is cheaper than coal, but has a high sulphur content, which opponents claim causes more pollution. It also contains heavy metals, like vanadium, which may be carcinogenic.

Environment Agency regulators have concluded that there is no evidence from Drax's petcoke trial to show an increased risk to human health or to the environment from burning or processing the fuel.

Gerard Morris, the agency's environment manager, said: "We are satisfied that all the results from the trial fall within environmental limits, and we will continue to monitor operations closely."

He said the power station, which supplies seven per cent of the National Grid's energy needs, will have to remain within all emission limits set in the operating permit.

"The trial showed a consistent small net environmental benefit from burning petcoke," he said.

"Petcoke combustion improved the efficiency of flue-gas clean-up equipment, and led to lower concentrations of dust being emitted from the station."

Drax has permission to burn up to 300,000 tonnes of specified types of petcoke every year. It can store up to 6,000 tonnes of petcoke blended with coal on the coal stockground.

Mr Morris said any requests for increases in these limits would have to be supported by monitoring data. "A steering group, including local stakeholder representatives and heath advisors, will scrutinise these requests," he said.

"Permission to burn petcoke can be withdrawn or restricted at any time if there is any evidence of significant threat to human health or the environment."

Coun Chris Metcalfe, chairman of the council's environment board, said: "The trial looked at a number of key issues, including dust levels in local villages and emissions levels. We think it's really important to go on checking these to fulfil our role in supporting and representing local residents.

"A clean, green and healthy environment is one of our priorities, so it's vital that we do all we can to ensure the burning of petcoke has no detrimental impact on any community within the district."