By James Morgan
Science reporter, BBC News
Many of the UK's coal plants are due for closure or replacement
Coal-fired power stations must not be built unless they can capture and store CO2, the Environment Agency has warned.
Plants like that proposed at Kingsnorth in Kent could "lock" the UK into high carbon technology, says the agency, whose remit covers England and Wales.
Even if stations can be fitted with clean coal technology at a later stage, that is "insufficient" for them to be approved now, the Agency argues.
The government will make its final decision on Kingsnorth within months.
Opponents fear that if the prime minister approves power company E.On's bid, it could pave the way for a whole series of similar coal plants.
"Building a new generation of coal-fired power stations without capturing the carbon emissions would lock the UK into using high carbon technology for decades to come," said Lord (Chris) Smith, chairman of the Environment Agency.
"This is not an environmentally sustainable way of generating power given the challenges we face with climate change.
"Although carbon capture and storage technology (CCS) has been demonstrated on a small scale, there is now an urgent need for it to be demonstrated on a commercial scale," he added.
"Any new coal power station to be built should have a consent that requires that it helps demonstrate the technology.
"Such a consent should be strictly time-limited and only renewed if carbon capture and storage is fully deployed."
CCS involves capturing the carbon dioxide emitted during the burning of fossil fuels, and then transporting it and storing it in a secure geological facility.
E.On says its new station at Kingsnorth will be built with "carbon capture readiness". That is, it will have the capability to install CCS technology at a later date, should it ever become commercially viable.
It could take decades for CCS to become cost effective
But the Environment Agency believes that guarantee is "insufficient" for the government to approve the power plant.
Before banking on new coal, we must see proof that CCS is viable, explains Lord Smith in response to a government consultation.
"We need to ensure that coal-fired power stations are part of a solution to the challenges of climate change, not a problem.
"This is only possible by ensuring carbon capture and storage is quickly proven in line with the Prime Minister's recent commitment to clean coal technology.
"A funding mechanism will be urgently needed to support this development."
The government has promised a decision in October on how it will fund a full-scale CCS proposal in the UK.
But even if the technology can be demonstrated, it may take decades before CCS becomes cost-effective, when compared to other methods of reducing emissions.
The heaviest polluting plants would not reach competitive levels until 2030, according to a new study by McKinsey & Company, a business consulting firm.
The current Kingsnorth power station is due to close in 2015. E.On wants to replace it with two new coal units, which it claims will be 20% cleaner.
Its proposals have already been approved by Medway Council. The final decision now rests with Business Secretary John Hutton.
Additional reporting by Roger Harrabin