A poster for British Midlands' no-frills carrier Bmibaby advertised one-way fares to Dublin or Murcia, in Spain, for £25, including taxes, when booked over the internet.
But the bottom of the advert said: "From East Midlands Airport, book online or call."
A customer who wanted to fly to Murcia contacted the ASA after telephoning Bmibaby and being told details about the advertised price were available only through the company's website.
British Midland confirmed the £25 fare was available only over the internet, and that prices booked by telephone started at £30.
Second complaint
After carrying out an investigation, the ASA said the poster failed to make the distinction clear and upheld the complaint.
It asked the airline to change the wording and told it to contact the advertising body set-up to prevent companies breaking the industry's code of practice.
The decision is the second complaint about Bmibaby upheld in the past three months.
It was criticised for an e-mail advertisement which did not make clear that travellers had to buy a return flight to get the quoted price.
Complaints to the ASA about no-frills airlines have reached 21 so far this year, up from 16 in the whole of 2001.
On Tuesday low-cost carrier Ryanair was criticised by the air passengers' watchdog the Air Transport Users' Council (AUC) for its standards of service.
Ryanair came third behind British Airways and Air France.
But the AUC said that it was especially worried about the budget airline which, it claimed, often displayed a poor attitude towards its customers.