A loophole in the UK Government's new energy bill will allow the coal-fired power station proposed for Hunterston in North Ayrshire to belch out hundreds of millions of tonnes of climate pollution and wreck Scotland's targets to cut emissions.

The hitherto unnoticed get-out clause was slipped in by Westminster after lobbying behind the scenes by the coal industry, and contradicts promises made by Prime Minister David Cameron and the Liberal Democrats.

It has been condemned as "utterly ridiculous" and "crazy" by leading environmental groups. Control over energy is not devolved to the Scottish Parliament or Government.

The loophole exempts coal stations that adopt government-approved technologies to trap carbon emissions from tough new limits on pollution. But the technologies – known as carbon capture and storage (CCS) – may only cover a proportion of the pollution, and may not work.

The £3 billion coal station planned for Hunterston by Ayrshire Power, set up by the property company, Peel Holdings, is intending to trap less than a fifth of its emissions with CCS technology. The plan, due to face a public inquiry later this year, has prompted over 20,000 objections, making it one of Scotland's most disputed planning applications.

The loophole was inserted in the UK energy bill's small print to appease coal companies anxious about their future. An internal industry memo in December to the UK Energy Minister, Charles Hendry, set out what they wanted in the bill.

Hendry was urged "to re-affirm the Government's interest in coal-fired electricity production" by adopting measures to maintain the annual 20-million-tonne market for coal. This could include offering "relief in some form" from new pollution limits for plants with CCS, the memo said.

"It is utterly ridiculous to propose giving this massive get-out-of-jail-free card to the power industry," said Dr Richard Dixon, the director of WWF Scotland. "With this gaping loophole a power station only has to fit a small green fig leaf of CCS and the rest of the station will escape proper climate controls for decades."

He estimated that the Hunterston plant, with only 17% of its carbon captured, would emit 320 million tonnes of carbon dioxide over its 40-year life. "Allowing it to escape carbon controls would pretty much guarantee that Scotland would miss its climate targets," he argued.

"This is a classic example of an anti-environment policy dressed up as being green. We need to defeat this crazy proposal."

Greenpeace accused the Conservative and LibDem Coalition of breaking its promise to end the era of dirty coal. "Some of the most polluting power stations known to man could be built in the UK and exempted from any pollution controls," warned the environmental group's senior campaigner, Joss Garman.

"If the industry's CCS experiments don't work, we'll be left with dirty new coal stations belching high levels of carbon emissions for decades into the future."

In a speech before the last election, David Cameron promised that "dirty energy has no future" under the Conservatives. The coalition agreement with the LibDems said that new coal plants would not be built unless they met the pollution limits.

The UK Government agreed that there was an exemption for CCS projects, but denied that it represented a loophole.

"The draft energy bill establishes the legislative framework for delivering secure, affordable and low-carbon energy including CCS," stated a spokesman for the Department of Energy and Climate Change.

"It is possible a very limited number of projects could be built as part of the CCS commercialisation programme that will not be subject to the emission-performance standard. This exemption will allow us the flexibility to select the best projects to take us to a new CCS industry as quickly as possible."

The spokesman pointed out that any new coal station would be subject to existing planning procedures. "We rightly have one of the most stringent consenting frameworks for coal in the world and any new coal plant will have to be built with CCS on at least 300 megawatts of generating capacity," he said.

The Scottish Government is studying the UK Government's draft energy bill. "As this is a live planning application it would be inappropriate to make further comment on this specific project," said a spokesman for Scottish ministers.