The BBC has found that several low-cost airlines are still quoting fares without including extras such as taxes. In February, the OFT gave carriers and travel companies three months to include all fixed nonoptional costs in their basic advertised prices.

Easyjet, Ryanair and Thomas Cook say they are working towards compliance. The extra costs which concern the OFT include items such as air passenger tax and fuel surcharges, all of which can significantly change the price of a flight.

Ray Hall, from the OFT, said advertising pricing in this manner was "leading consumers to make the wrong choices."

"We have given the airline industry a deadline to make changes . . . some have complied, some haven't, " he said.

The OFT said a minority of airlines had failed to comply, although it will not name them.

However, several low-cost carriers are still quoting flight prices without including those extras on their websites.

Easyjet said that by the end of the summer all prices on its website would include the noted extras. It added its other advertisements . . . in the press and on billboards . . . were compliant.

Ryanair said its entire website would be compliant by the end of the year. Flythomascook has set a deadline for the end of July and Jet2 said it was talking to the OFT about how best to present its prices.

If the OFT is not happy with the rate of progress, it can take companies to court to force them to change.