A Liberal Democrat government would oppose expansion at major airports such as Birmingham but back plans for a new high speed rail line instead, the party said yesterday.

Transport spokesman Susan Kramer said it was "madness" that passengers were flying between London and Birmingham.

More than one in four flights from Birmingham International are domestic flights.

Speaking to delegates at the party's annual conference, she declared: "I say this to the industry - we will not support an increase in runway capacity at major airports."

Birmingham International Airport is planning a 400-metre runway extension across the A45, which will enable long haul flights to travel to China and the west coast of America without stopping en-route to re-fuel.

The expansion is backed by local authorities including Birmingham City Council, which claims the benefit to the regional economy from a longer runway is likely to be £1 billion and 16,000 new jobs by 2030.

A Liberal Democrat policy paper, published last month and approved by delegates yesterday, calls on a Lib Dem government to "retain runway capacity in major airports at around the current level".

However, Birmingham MP John Hemming insisted yesterday that the party was not opposed to expansion at Birmingham International.

He said "We opposed the idea of a second runway at Birmingham when that was being discussed. But the airport itself has accepted that a second runway is not needed at the moment.

"That doesn't mean no expansion of the runway that already exists."

Ms Kramer said the party would impose an extra £10 tax on any flight which both began and ended in the UK, in a bid to reduce the number of domestic flights.

This would raise around £200 million a year, which would help pay for major rail projects including a new high speed rail link.

A high speed line would be expected to run from London to Scotland via Birmingham and Manchester, with trains reaching speeds of 225 mph. It could cost £14 billion.

Earlier this week, regional development agencies in the North-west, North-east and Yorkshire and Humberside launched a joint campaign in support of a high speed line.

Speaking at the conference, Ms Kramer said the growth of the air industry had to be stopped.

"If we do nothing, aviation will soon rival cars and lorries for the accolade of the worst polluter.

"If we continue 'business as usual', air travel will account for a quarter of the UK's contribution to global warming in 30 years."

She added: "I don't know about you, but after my experiences at British airports this summer, and after the farcical attempt to treat climate change protesters like terror-ists, any charitable instincts I may have had towards the aviation industry are long gone."

Domestic flights were the fastest growing sector of the aviation industry, she said.

"It is genuine madness that any traveller has to turn to plane travel to get from London to Birmingham or Glasgow to London in a reasonable time at a reasonable price."