Come Tuesday, Delta Air Lines and UAL Corp's United Airlines will link their frequent flier programmes, the latest partnership among the wave of alliances being formed by the nation's biggest airlines.

While the airlines say the programmes are creating a new world of air travel, some consumer groups say they are creating confusion and believe passengers could have a harder time redeeming mileage points.

``The biggest problem is going to be many, many more people chasing limited inventory,'' said president Kevin Mitchell of Business Travel Coalition, a group representing small business travellers.

Existing frequent flier programmes, which were pioneered in the early 1980s by AMR Corp's American Airlines, allow passengers opportunities to earn free tickets by racking up points for travel on a particular airline.

The partnerships beingformed expand the programmes. The Delta-United partnership will allow Delta passengers to accrue points from flights they take on United, and vice-versa.

American and US Airways, which have been phasing in their link-up since August 1, are letting passengers combine miles to redeem travel on either airline.

Airlines say their new partnerships make it easier for passengers to earn points and increases the places where they can fly onward miles by giving them access to a partner's destinations.

``Our members actually have over 95 new paces to go to,'' said vice-president Martin White of marketing programmes and services at US Airways.

He said American passengers will have a better chance of redeeming awards for travel in Florida and Europe, and US Airways passengers will have a shot at places American flies, such as Hawaii, Japan and South America.

Not so fast, consumer groups contend.

They say linked programmes could increase the number of people competing for choice destinations, such as Hawaii,and that the benefits of new destinations are not as generous as the airlines are promoting.

``Boise, Idaho, which is served by United, is not going to be that exciting for people in Delta's new programme,'' said publisher Randy Petersen of Inside Flyer Magazine.

They also say there could be long-term pitfalls, including the possibility that airlines will raise the number of points required for free tickets as they try to manage a ballooning number passengers trying to redeem bonus travel.

``In reality the airlines control the usage of frequent flier miles, as anybody who's tried to get a ticket knows,'' said, an airline analyst Thomas Longman with Arnold S Bleichroeder.

While the programmes could make it easier to earn frequent flier miles, Longman said, airlines could make the miles harder to use as more passengers try to redeem their miles under increasingly complicated rules.

President Bruce Chemel of American Airlines' Advantage marketing division, said airlines have a financial incentive tomake the programmes accessible.

He said American, whose 33 million frequent flier members give it the biggest award programme, has 2,500 partners who buy mileage points from American to use as marketing tools to attract customers in their own promotions.

This doesn't even include the airline's biggest partners, such as long-distance phone and credit card companies.

Although American Airlines declined to comment on how much it earns by selling points to third parties, Petersen estimates the airline earned as much as $500 million from selling mileage points to outside parties last year.