Fears for the state of the UK aviation industry deepened after National Express warned that fewer people are travelling to Stansted, hitherto London's fastest growing airport.

In a trading update today, National Express warned twice of the effect that Stansted is having on its business: of a "softening of demand" on its Stansted Express train service out of Liverpool Street and of "lower footfall" on coach services to the Essex airport.

The news from National Express will set the alarm bells ringing for investors in easyJet and Ryanair, the two budget airlines responsible for Stansted's huge success in recent years.

Stansted routinely grew passenger numbers at 25% per annum to the point where it now handles 23 million passengers a year. However, the recession in the aviation industry has already led to a 4.3% fall in passenger numbers at the airport in the first quarter of 2008.

Indications that performance has weakened into April will weigh heavily on the two budget carriers, whose shares have both halved over the last 12 months. Both have issued profit warnings since the start of the year - with their bottom lines further damaged by the soaring cost of aviation fuel - and many analysts believe further alerts.

Those profit warnings could come as early as next week, when the two carriers report their April traffic statistics.

National Express said that the fall in Stansted demand held back revenue growth in its trains business to 9% in the four months to the end of April, a slower performance than that reported recently by rival commuter train companies Stagecoach and Go-Ahead.