* Alex Pabon loses case in London's Court of Appeal
* Pabon was convicted of Libor rigging in 2016
* Judges call appointment of SFO witness a "debacle" (Adds SFO comment, police comment, judgment details)
LONDON, March 13 (Reuters) - A former Barclays trader lost an appeal against his conviction over Libor benchmark interest rate rigging at London's Court of Appeal on Tuesday.
Alex Pabon, a 39-year-old American former derivatives trader, was jailed in 2016 alongside three other former Barclays traders for conspiracy to defraud in relation to the manipulation of Libor (London interbank offered rate).
But Pabon, who was released last March after serving just under a year in prison, brought an appeal after new evidence came to light about the expertise of one the Serious Fraud Office (SFO) prosecutor's key banking witnesses in the case.
Judges at London's Court of Appeal dismissed the appeal. But in a strongly-worded judgment, they also criticised SFO's decision to instruct Saul Haydon Rowe as a court witness.
"The instruction of Rowe turned into an embarrassing debacle for the SFO ... (although) it has had no impact on the outcome of this case," the judgement said. It added that the case was a "stark reminder" that those needing expert witnesses should ensure they were instructing people of "suitable calibre".
Rowe came in for the harshest criticism. The judges said he had "signally failed to comply with his basic duties as an expert". But they ruled that his evidence had not impacted sufficiently on Pabon's trial to affect the safety of his conviction.
Pabon's lawyers had argued that the evidence Rowe gave during Pabon's trial was incomplete or inaccurate and could have damaged the trader's credibility.
Rowe has previously told Reuters he had always acted in good faith. He did not respond to telephone calls requesting a comment on Tuesday.
Pabon's lawyer was not immediately available for comment. But Pabon said on social media site Twitter: "There is no such thing as an expert witness in the UK anymore."
The SFO said it was "studying the judgment carefully". Barclays declined to comment.
Rowe, a former trader who ran a company providing expert consulting and testimony in banking cases, was paid more than 400,000 pounds ($557,000) between 2014 and 2017 by the SFO to be a court expert on banking in its Libor prosecutions.
He testified in four Libor trials by providing expert evidence on banking and trading and interpreted communications with or between rate derivatives traders presented in court.
Judges said his failings included obscuring the role others had played in a key report for the prosecution, failing to inform the SFO about the limits of his expertise and flouting court rules about not discussing evidence during testimony breaks.
"We take a grave view of Rowe's conduct; questions of sanction are not for us, so we say no more of sanction but highlight his failings here for the consideration of others," the judgment said.
At least three former Libor traders have complained about Rowe to the London police. The Metropolitan police says it continues to assess complaints about witness testimony.
Pabon's conviction is part of a global investigation into allegations that banks colluded in setting benchmark rates such as Libor, against which rates on hundreds of trillions of dollars worth of contracts and loans are set across the world.
Around a dozen of the world's largest banks have been fined about $9 billion pounds as part of the investigation and about 30 traders have been charged in Britain and United States.
($1 = 0.7182 pounds) (editing by Simon Jessop, John Stonestreet and Jane Merriman)