Ryanair has accused advertising watchdogs of "vexatious meddling and interference" after the low cost airline was criticised for sending out misleading emails to British customers.

The row broke out after the Advertising Standards Authority of Ireland ruled in favour of a complaint by leading ferry operators that the airline's policy of sending out emails pricing flights without the additional taxes and charges flouted its code of practice.

While the ASAI admitted that policing such marketing emails was problematic, it insisted that electronic marketing fell within its remit.

"The complaints committee in previous situations had confirmed that email and printed commercial communications sent by firms to customers and other interested parties who have asked for details of future products and services and direct mailings sent with or without request are all subject to the ASAI codes," the regulator said in its ruling against the airline.

"The code applies to advertisements carried on audio tapes, videotapes, view data services, the internet and all other electronic computer systems."

Ryanair criticised the ASAI for acting illegimately, saying the emails were "private correspondence" between itself and customers who have specifically opted to receive an update service.

"This correspondence is personal, private and confidential and is beyond the jurisdiction of the Advertising Standards Authority of Ireland," the company said in a statement.

"It has already been accepted by the Advertising Standards Authority that it is beyond their jurisdiction but the ASAI continues to behave in an illegitimate manner by pursuing an issue which is clearly beyond its terms of reference."

The low cost airline, which prides itself on an advertising philosophy that "now and again upsets establishment", said it had received no complaints from any of its customers and dismissed the ruling as an attempt by its rivals to gain the upper hand.

"Consequently Ryanair does not regard this meddling and interference by the ASAI as anything other than vexatious and entirely irrelevant."

The ASAI said it was "surprised and disappointed" at the stand taken by Ryanair and requested the airline to comply with the code.

Meanwhile the ferry groups that brought the complaint welcomed the ruling as a milestone in their campaign.

"This is another step forward in our fight to level the playing field in the transport industry," said Bill Gibbons, the director of the Passenger Shipping Association.

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