Long-term demand for air travel could be even higher than previously thought because of the dramatic growth of no-frills carriers.

A confidential Government study counters claims from opponents of airport expansion that fears of terrorism, economic uncertainty and environmental taxes will keep passenger numbers below levels at which extra runways would be required.

The Department for Transport's last official forecasts, compiled three years ago, predicted that demand would rise from 180 million passengers a year to 500 million by 2030.

But a fresh analysis, circulated to senior industry figures, concludes that the impact of budget airlines and the competitive response of some traditional carriers "could push demand beyond" the earlier projection.

It reveals that users of low-cost operators last year totalled more than 35 million, nearly a five-fold increase on the 1998 figure. The estimates published in 2000 stated that the no-frills market would not exceed 30 million until 2020.