LONDON (Reuters) - Advertising watchdog on Wednesday criticised Irish airline Ryanair for promoting millions of "zero fare" flights that actually cost passengers up to 21.70 pounds.

The Advertising Standards Authority (ASA) said the low-cost airline should have made clear that people would have to pay taxes and other charges on all tickets.

It upheld a complaint from Ryanair's rival Monarch Airlines and ordered the Dublin-based company to drop the TV commercials.

In a written adjudication, the ASA said Ryanair breached its code because the charges on each ticket were "non-optional".

Ryanair's "amazing autumn sale" commercials promised "Four Million Zero Fare Seats". Small print at the bottom of the screen said taxes and charges applied.

Ryanair had no immediate comment.

The airline has fallen foul of the ASA before.

In May, the watchdog upheld a complaint about an advert for three million free tickets because it failed to make clear that the offer did not include flights on Fridays.

In 2004, it rapped a promotion offering "giveaway" flights because passengers would have to pay charges and taxes.

The European Union's top regulator said in July it planned to clamp down on misleading air fares.

Under the European Commission proposals, airlines would have to be more transparent about the final price passengers pay.