Colin Bermingham, a former cash market expert and veteran banker, and Carlo Palombo, who once traded derivatives, were last year sentenced to five and four years respectively in the UK Serious Fraud Office's (SFO) sixth rate-rigging prosecution.

"We are very disappointed at the ruling," said Raj Chada, head of criminal defence at Hodge Jones & Allen Solicitors, who represents Palombo.

"It remains clear to us that Mr Palombo's conviction is unsafe and we are considering our options for a further appeal."

The appeal hinged partly on whether trial judge Michael Gledhill was right not to investigate one juror's financial knowledge, or his former role at UBS, and whether he should have given defence lawyers an opportunity to object to the juror being on the panel.

Lawyers for the men said the juror may have had knowledge that could have influenced other jurors and risked prejudicing the verdict.

But Court of Appeal judges said they did not believe there was a real possibility that the juror - who worked as an intern at UBS four years after the end of the indictment period and nearly six years before serving on the jury - was biased, equally dismissing other appeal grounds.

There was no prima facie evidence that the juror behaved inappropriately either by conducting internet searches or sharing information that went beyond his professional experience or general knowledge, senior judges said.

David Corker, a partner at law firm Corker Binning, said allegations of juror bias are increasing, but that the judgment helped preserve the principle of random selection of jurors - unless they had more than a general knowledge.

Prosecutors alleged Bermingham and Palombo dishonestly manipulated Euribor - a benchmark that helps determine interest rates on about $180 trillion of financial contracts and loans worldwide - by requesting or submitting "false" rates that took account of commercial interests between 2005 and 2009.

Both men conceded they had made or taken into account trader rate requests justified by market rates. But they denied dishonesty, said they had acted openly and that bosses were aware of the widespread practice.

An SFO spokesperson welcomed the judgment, saying the men's actions had undermined a system "critical to the functioning of the global economy and risked the investments, savings and pensions of millions of hard-working people".

($1 = 0.7435 pounds)

(Reporting by Kirstin Ridley; Editing by Louise Heavens, Jan Harvey and Barbara Lewis)

By Kirstin Ridley