Return tickets will cost $2 extra under the scheme, designed to offset its ticket distribution costs which rose to $1bn last year and passed fuel as its second-largest expense.

Up till now, America's third largest airline has mirrored its competitors online, selling less than two per cent of its tickets through its site.

Around 2.5 per cent of all air tickets are estimated to have been sold online last year with the figure expected to rise to 8.4 per cent in 2002.

Net savings of online booking

Sales done over the Web save on computer reservation system booking costs and an 8 per cent commission paid to travel agents.

Many airlines offer cheaper seats online and have carried out auctions of unsold seats. But Delta's decision to penalise those buying through channels other than its Website is a first.

Delta said it was prompted by three of the four major computer reservation companies raising their fees by nearly three per cent to 5.2 per cent at the start of this year.