The warning came from O'Leary's arch rival in lowcost airlines, Easyjet founder Stelios Haji-Ioannou, as he addressed a gathering of senior UK marketers in London on Thursday.

"O'Leary is very successful at managing expectations, " Stelios told guests at the dinner. "He carries 40 million customers every year with very little by way of customer promise. He is very successful at lowering [customer] expectations. But denigrating your product is not allowable, according to the marketing textbooks, and whether O'Leary can continue doing so I don't know."

However, Stelios said he was "an admirer" of O'Leary, who has transformed Ryanair into Europe's most profitable carrier. "He is very successful financially . . . you don't get to be a tax consultant at KPMG without being good with numbers . . . but I'm not so sure it's any excuse to be so arrogant."

The pair have had an uneasy relationship since O'Leary began Ryanair's conversion into a 'no-frills' airline in the mid 1990s. It meant competing with Easyjet, which Stelios founded in 1995 as the pioneer of UK low-cost air travel.

Since then, Stelios and O'Leary have matched each other for publicity-seeking stunts. In 2003, O'Leary drove a second World War tank to Luton airport, and with the theme to the TV series The A-Team blaring, declared he was "liberating the public from Easyjet's high fares".

For his part, Stelios once boarded another rival's launch flight wearing a bright orange jump suit, handing out free Easyjet tickets to passengers.