PARIS, Dec 20 (Reuters) - Qatar Airways said on Monday it had started proceedings in a UK court against planemaker Airbus (AIR.PA) in a bid to resolve a dispute over skin flaws on A350 passenger jets, bringing the two sides closer to a rare legal showdown over aviation safety. The companies have been locked in a row for months over damage including blistered paint and corrosion to a sub-layer of lightning protection, which Qatar Airways says has now led to the grounding of 21 A350 jets by its domestic regulator. Airbus insists the carbon-composite passenger jets are safe to fly despite some "surface degradation," while Qatar Airways says it is too early to say whether safety has been compromised. The dispute came to a head last week when Airbus, in what experts called an unprecedented move, accused the Gulf airline of misrepresenting the problem as a safety issue and threatened to call for an independent legal assessment. read more On Monday, Qatar Airways hit back, saying it had taken its complaint against Airbus to the High Court in London. "We have sadly failed in all our attempts to reach a constructive solution with Airbus in relation to the accelerated surface degradation condition adversely impacting the Airbus A350 aircraft," it said in a statement. "Qatar Airways has therefore been left with no alternative but to seek a rapid resolution of this dispute via the courts." Airbus said it had nothing to add to an earlier statement that it had already found the cause of the problem. It reiterated it was working with Europe"s safety regulator, which has said analysis to date has not identified any safety issue. "We are working with the customers in order to provide enhancements," an Airbus spokesperson said. Qatar Airways denies that the surface flaws - which witnesses say have left some of the jets with a pock-marked appearance - are properly understood and said on Monday that it wanted Airbus to mount a "thorough investigation". read more BIGGEST ORDER The row widened this month when documents seen by Reuters revealed at least five other airlines located in varying climates had complained about paint or other surface problems since 2016. Airbus has until recently maintained the problem was focused on paint on Qatar"s jets, operated in desert conditions. Reuters also first reported that Airbus was looking at changing the anti-lightning system. read more The planemaker has said it is proposing interim solutions ranging from repairs to repainting and has accused Qatar Airways of ignoring those proposals without reasonable justification. Qatar Airways reiterated on Monday it could not be sure whether proposed repairs would work without deeper analysis. The 21 grounded jets represent 40% of its current fleet of A350s, for which it was the launch customer with the biggest order. Other airlines still operate the jet, saying its airworthiness is not affected by what they term cosmetic issues. The row meanwhile looks set to cost Airbus a potentially major order from Qatar for a new A350 freighter version. It received the first firm order for the new model earlier on Monday, confirming a previously tentative order for four planes from France"s CMA CGM. Qatar Airways Chief Executive Akbar Al Baker told the South China Morning Post last week he had been looking at placing a large order for the new cargo A350. Sources expect Boeing to land an order to replace Qatar"s 34-35 freighters. read more