RYANAIR boss Michael O'Leary has dismissed the Thames Estuary airport idea, backed by Boris Johnson, as "insane, stupid and hare-brained".

The budget airline chief made the stinging attack during evidence to the House of Commons transport select committee, warning his language about the proposals could be "unsuitable" for Parliament.

When the subject of the hub, dubbed 'Boris Island' because of the Mayor of London's support, was raised by MPs Mr O'Leary told the committee: "Such a scheme is insane, stupid and hare-brained.

"To try to come up with a new green-field site plan with no infrastructure and no motorway or rail links and which would take the rest of my life to build is insanely and asininely stupid."

A design by architect Lord Norman Foster for a £50bn new Thames hub, including a massive new airport on the Isle of Grain with capacity for 150 million passengers annually, was released last year.

But Mr O'Leary, famed for his forthright opinions, told MPs he backed new runways at London's existing airports instead, claiming the government was simply "kicking the aviation strategy can further down the road" with the formation of an independent commission to examine expansion.

The latest criticism comes just after Prime Minister David Cameron rebuked the Mayor of London over his increasing calls for the airport to be built.

But Mr Johnson repeated his advocacy for the plan again this week, telling the annual CBI business conference that only the Thames Estuary airport would enable the UK to increase growth and boost jobs in the south east.