Nick Clegg says he's "unpersuaded" of the need to build the "Boris Island" airport in the Thames Estuary, confirming that the issue could be a source of tension in the coalition.

Speaking on the Andrew Marr show on BBC One, Clegg said: "I'm totally un-persuaded by the evidence and think we should take a common-sense, hard-headed look at the facts. I don't think we should as a country decide to concrete over vast swathes of the Thames Estuary on a whim.”

Clegg said he would be happy to look at the results of a wide-ranging consultation on UK aviation, due to begin in March, but sounded highly sceptical about the mooted Thames plan.

"As it happens if you look at the facts, we have four big airports around London, three of which aren't even being used to capacity yet. Call me old-fashioned but it seems like you should first look at things like that."

The DPM was referring to Stansted, Luton and Gatwick airports, which have spare capacity. Heathrow regularly runs at near-capacity and is also subject to fog problems and noise restrictions due to its proximity to central London.

The Liberal Democrats went into the 2010 general election opposing any new airports in the south-east of England, and there is no provision within the coalition agreement to build one.

Earlier this week reports appeared suggesting David Cameron and George Osborne had come round to the idea of building a major airport somewhere east of London, a plan dubbed "Boris Island" because the Mayor of London has been a long-time proponent of constructing one in the middle of the Thames Estuary.

Since then there have been a series of contradictory reports. Some believe the Tories will attempt to re-negotiate the coalition agreement in order to include the new airport. But The Guardian says that Boris Johnson leaked the original story of the PM being supportive of the airport, and that doing this has in effect scuppered the plan.

In addition to Liberal Democrat opposition, the government would also have to deal with gripes from many Tory MPs in the region, who can't see the benefits of a new airport.