A senior Conservative has called on David Cameron to decide if he is “a man or mouse” and agree to a third runway at Heathrow Airport.

Tim Yeo, who is a former environment minister, used to be opposed to the expansion of Heathrow but now says the environmental objections to it are disappearing.

Writing in The Daily Telegraph, the chairman of the Commons energy committee claimed backing the expansion would give the Government a “sense of mission”.

In the stinging attack, he questioned whether the Prime Minister would preside “over a dignified slide towards insignificance” and suggested the leader’s heart was “impenetrable” to most voters.

“The Prime Minister must ask himself whether he is man or mouse,” Mr Yeo said. “Does he want to be another Harold Macmillan, presiding over a dignified slide towards insignificance?

“Or is there somewhere inside his heart – an organ that still remains impenetrable to most Britons – a trace of (former PM Margaret) Thatcher, determined to reverse the direction of our ship?

“An immediate go-ahead for a third runway will symbolise the start of a new era, the moment the Cameron government found its sense of mission. Let’s go for it.”

It comes after housing minister Grant Shapps warned a third runway was needed to ensure the UK remained a “great trading nation”.

London mayor Boris Johnson, who is against the new runway, has accused Mr Cameron of “pussyfooting around” on the issue.

Mr Yeo was previously a high-profile opponent of expansion but now argues European Union carbon emissions caps will force airlines to use more environmentally friendly planes, if they want to use new capacity at Heathrow.

He added: “The environmental objections are disappearing. Last January, greenhouse gas emissions from flying were brought within the EU cap.

“Indeed, we could cover the whole of Surrey with runways and not increase emissions by a single kilogram: if Heathrow expands, so remaining the European destination of choice, airlines will fly their newest and quietest aircraft to it.”

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