The new head of the UK government's official green watchdog has strongly criticised moves to build new coal-fired power stations in Britain and condemned the planned expansion of Heathrow.

In his first major interview since taking office, Will Day, the incoming chair of the Sustainable Development Commission (SDC), told the Guardian that construction of new coal stations, such as the planned Eon Kingsnorth facility in Kent, would provide a "lightning rod" for international protest.

He dismissed industry and ministerial claims that new power stations such as Kingsnorth could be operated with limited impact on the environment by trapping and storing the carbon emissions underground. "Never use the words 'clean coal'," he said. "I do not believe clean coal exists."

Day also said that:

• It was "not an appropriate time" to build a third runway at Heathrow.

• Flights must be made more expensive to discourage people from flying to foreign holidays.

• Politicians must make unpopular decisions to tackle global warming

Day stressed his views were personal and not those of the SDC, but their uncompromising tone is likely to reassure green campaigners. Day, the former chief executive of relief and development group Care International, was appointed SDC chair following the departure of Jonathan Porritt, who was a regular and vocal critic of government efforts on the environment.

On new coal power stations, Day said: "Science is unequivocal about the impact of carbon on our environment. Every time scientists go back to measure ice and water levels and those things it gets worse. We should not be adding to that problem. And when someone says "oh no, it will be carbon capture and storage ready", well show me where it's working, seriously working. Show me how it's going to be implemented on existing stock, let alone new stock."

He added: "There is no such thing as a free lunch and we're not going to get a free lunch around coal. So my view would be if the government wants to provide a lightning rod for public disagreement or dissent around coal, then start building a new coal-fired power station, and the orang-utan costumes will be dusted off from around the planet and people will come and say this is wrong. And two wrongs don't make a right. People say "oh there is one a week opening in China". And? I don't think that's a good enough reason."

Day said he disagreed with Ed Miliband, climate and energy secretary, on whether mass air travel could be preserved in a low-carbon world. Miliband told the Guardian last month: "Where I disagree with other people on aviation is if you did 80% cuts across the board, as some people have called for on aviation, you would go back to 1974 levels of flying. I don't want to have a situation where only rich people can afford to fly."

Day said: "Politicians are there to make the hard decisions. And there are some really hard decisions coming up. And they're hard because they're not the kind of decisions that individuals particularly want to have taken. How many short- to medium-haul flight holidays does anyone really need to have a year? Ed Miliband interestingly said something like 'don't worry your holiday flights are safe with me'. But we know that we need to be encouraging and supporting, through a combination of stick and carrot, some change to behaviours."

He added: "Part of the difficult decision is going to be a rebalancing of what things cost. If we say we must pay the true price of the impact of carbon on the environment. The hard decision is do you price the impact of an aeroplane flying through the air properly, really properly, and not a kind of £1.20 carbon offset. The objective is to reduce the amount of carbon put into the upper atmosphere by planes by pricing it out."

He said: [Flights] will continue but there will be fewer of them, and they will be properly priced. And people will be able to make decisions based on their decision to afford. They're not being told they cannot go on holiday, they are being told this is what it costs."