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The argument kicked off yesterday (15 January) evening. The tone of Ryanair's tweets suggest short tempers and temporary amnesia have affected its ".

The squabble commenced after Aer Lingus spotted one its aircraft in the background of a Ryanair ad, albeit with the Aer Lingus branding only apparent to the eagle-eyed.

Aer Lingus tweeted: "Epic #photobomb… on Ryanair’s website! Even they know we’re Best for Business".

Epic by EI-DVM on Ryanair's website! Even they know we're Best for Business ??

But Ryanair, the budget airline fronted by fiery chief executive Michael O’Leary, which has recently promised to take a less combative approach to business, retorted: "Even @AerLingus is talking about @Ryanair" and, referring to the smiling man in the ad, wrote: "FYI, he’s smiling cos he saved a fortune & avoided another late @AerLingus arrival!"

Even is talking about Business Plus. FYI, he's smiling cos he saved a fortune & avoided another late arrival!

— Ryanair (@Ryanair)

The contretemps escalated after a Twitter user asked if the airlines hated one another. Aer Lingus graciously described the relationship as a "healthy rivalry" and asked its competitor to agree, but Ryanair was quick to set the record straight.

Far less graciously, it tweeted: "We stopped being rival in the 90s @AerLingus… We’d have to treble our fares & lose 81m customers to be your rival".

We stopped being rivals in the 90s . We'd have to treble our fares & lose 81m customers to be your rival

Aer Lingus did not rise to Ryanair's levels of invective, and responded: "Okay, nevermind".

Meanwhile, Ryanair will find out today whether or not it will be forced to sell its near-30% stake in Aer Lingus, with a decision due from the UK Court of Appeal.

The airline has appealed on several occasions after being told by regulators that it must reduce its stake to 5%.