NICK Clegg has suffered a humiliating blow after Liberal Democrat activists refused to bow to leadership pressure and relax their opposition to airport expansion.

However, the party leader, who branded the conference decision nonsensical, made clear the issue would "need to be discussed again" because blanket rejection of airport expansion was not a deliverable policy.

He was hoping members would agree to the possibility of building extra runways at Gatwick and Stansted - expansion at Heathrow has already been ruled out.

Energy Secretary Ed Davey had earlier appealed for the Lib Dem rank-and-file to endorse the proposed change, arguing that improvements to aircraft technology meant it was possible to expand some airports without endangering the environment.

But, with Mr Clegg in the hall, activists voted to maintain the current policy, which specifies "no net increase in runways across the UK as a whole".

The leadership is thought to have been keen for the party to give itself wriggle room on the airports issue, should there be another Coalition after the May 2015 General Election, rather than risk being forced into another tuition fees-style U-turn.

Following the vote, the Deputy Prime Minister insisted the Liberal Democrats belonged to a democratic party and were "not a sort of North Korean sect that does exactly what the leader wants". Clearly annoyed by the members' decision, he said: "I've got to be open with you, I just don't think it makes sense to say you're never going to have a single metre of extra runway anywhere in the UK.

"But it will need to be discussed again because, how can I put it, I've seen the perils of putting forward something which you know in your heart of hearts is not necessarily deliverable."

Heathrow and Gatwick are shortlisted as options for another runway by 2030.